feat: anthropic adapter with bidirectional translation incl streaming

Implements Task 9: the anthropic-type providerAdapter and its
request/response/stream translation to and from OpenAI's wire shape.

- translate_anthropic.go: anthropicRequestFromOpenAI (system messages
  joined, content blocks incl. base64 data-URI images, tool_calls ->
  tool_use, tool messages -> tool_result, max_tokens defaulting,
  stop/stop_sequences, tool_choice incl. the none-drops-tools rule,
  n>1/logit_bias/logprobs rejected), openAIResponseFromAnthropic
  (content blocks joined/tool_calls re-marshaled, stop_reason mapping,
  usage), and anthropicStreamState.translate mapping each SSE event to
  zero or more OpenAI chat.completion.chunk payloads. translateError
  carries a notSupported flag so the caller (Task 12) can tell a 400
  from a 501.
- provider_anthropic.go: anthropicAdapter implementing providerAdapter.
  newAnthropicAdapter errors on an empty API key per ruling (c), unlike
  openai-type's keyless ruling. Headers: x-api-key, anthropic-version.
  embeddings always returns a notSupported translateError without
  calling upstream (Anthropic has no embeddings endpoint). listModels
  GETs /v1/models and returns any non-2xx (incl. 404) as-is.
- providers.go: wires the anthropic buildAdapters case to
  newAnthropicAdapter, propagating its constructor error.
- provider_openai_test.go: replaces the now-stale placeholder
  TestBuildAdapters subtest with one asserting a keyed anthropic
  provider builds successfully and a keyless one errors.
- translate_anthropic_test.go: golden-file harness walking
  testdata/anthropic/{chat,resp}_*_in.json against sibling _want.json
  (success) or _wanterr.json (*translateError) fixtures, plus a stream
  harness driving each *.sse fixture through anthropicStreamState and
  asserting the chunk list and usage().
- provider_anthropic_test.go: httptest-backed adapter wiring tests
  (streaming + non-streaming), asserting OpenAI-shaped output on the
  wire, per-event flushing, and the required auth headers.

Gate: go test ./... -count=1 and -race both green; gofmt, go vet,
fieldalignment, golangci-lint (incl. gosec) all clean.

testdata/anthropic/chat_basic_in.json

@@ -0,0 +1,10 @@
+{
+  "model": "claude-opus-5",
+  "messages": [
+    {"role": "user", "content": "Hello, how are you?"}
+  ],
+  "temperature": 0.7,
+  "top_p": 0.9,
+  "stop": "STOP",
+  "user": "user-123"
+}

testdata/anthropic/chat_basic_want.json

@@ -0,0 +1,11 @@
+{
+  "model": "claude-opus-5",
+  "messages": [
+    {"role": "user", "content": "Hello, how are you?"}
+  ],
+  "max_tokens": 4096,
+  "temperature": 0.7,
+  "top_p": 0.9,
+  "stop_sequences": ["STOP"],
+  "metadata": {"user_id": "user-123"}
+}

testdata/anthropic/chat_images_http_error_in.json

@@ -0,0 +1,10 @@
+{
+  "model": "claude-opus-5",
+  "messages": [
+    {"role": "user", "content": [
+      {"type": "text", "text": "What is in this image?"},
+      {"type": "image_url", "image_url": {"url": "https://example.com/cat.png"}}
+    ]}
+  ],
+  "max_tokens": 300
+}

testdata/anthropic/chat_images_http_error_wanterr.json

@@ -0,0 +1 @@
+{"errContains": "data: URI", "notSupported": false}

testdata/anthropic/chat_images_in.json

@@ -0,0 +1,10 @@
+{
+  "model": "claude-opus-5",
+  "messages": [
+    {"role": "user", "content": [
+      {"type": "text", "text": "What is in this image?"},
+      {"type": "image_url", "image_url": {"url": "data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAEAAAAB"}}
+    ]}
+  ],
+  "max_tokens": 300
+}

testdata/anthropic/chat_images_want.json

@@ -0,0 +1,10 @@
+{
+  "model": "claude-opus-5",
+  "messages": [
+    {"role": "user", "content": [
+      {"type": "text", "text": "What is in this image?"},
+      {"type": "image", "source": {"type": "base64", "media_type": "image/png", "data": "iVBORw0KGgoAAAANSUhEUgAAAAEAAAAB"}}
+    ]}
+  ],
+  "max_tokens": 300
+}

testdata/anthropic/chat_system_tools_in.json

@@ -0,0 +1,18 @@
+{
+  "model": "claude-opus-5",
+  "messages": [
+    {"role": "system", "content": "You are a helpful assistant."},
+    {"role": "system", "content": "Always be concise."},
+    {"role": "user", "content": "What's the weather in Paris?"},
+    {"role": "assistant", "content": null, "tool_calls": [
+      {"id": "call_abc123", "type": "function", "function": {"name": "get_weather", "arguments": "{\"location\":\"Paris\"}"}}
+    ]},
+    {"role": "tool", "tool_call_id": "call_abc123", "content": "72F and sunny"}
+  ],
+  "max_tokens": 512,
+  "stop": ["END", "STOP"],
+  "tools": [
+    {"type": "function", "function": {"name": "get_weather", "description": "Get current weather for a location", "parameters": {"type": "object", "properties": {"location": {"type": "string"}}, "required": ["location"]}}}
+  ],
+  "tool_choice": "auto"
+}

testdata/anthropic/chat_system_tools_want.json

@@ -0,0 +1,19 @@
+{
+  "model": "claude-opus-5",
+  "system": "You are a helpful assistant.\n\nAlways be concise.",
+  "messages": [
+    {"role": "user", "content": "What's the weather in Paris?"},
+    {"role": "assistant", "content": [
+      {"type": "tool_use", "id": "call_abc123", "name": "get_weather", "input": {"location": "Paris"}}
+    ]},
+    {"role": "user", "content": [
+      {"type": "tool_result", "tool_use_id": "call_abc123", "content": "72F and sunny"}
+    ]}
+  ],
+  "max_tokens": 512,
+  "stop_sequences": ["END", "STOP"],
+  "tools": [
+    {"name": "get_weather", "description": "Get current weather for a location", "input_schema": {"type": "object", "properties": {"location": {"type": "string"}}, "required": ["location"]}}
+  ],
+  "tool_choice": {"type": "auto"}
+}

testdata/anthropic/chat_tool_choice_none_in.json

@@ -0,0 +1,9 @@
+{
+  "model": "claude-opus-5",
+  "messages": [{"role": "user", "content": "Do something"}],
+  "max_tokens": 100,
+  "tools": [
+    {"type": "function", "function": {"name": "noop", "description": "does nothing", "parameters": {"type": "object", "properties": {}}}}
+  ],
+  "tool_choice": "none"
+}

testdata/anthropic/chat_tool_choice_none_want.json

@@ -0,0 +1,5 @@
+{
+  "model": "claude-opus-5",
+  "messages": [{"role": "user", "content": "Do something"}],
+  "max_tokens": 100
+}

testdata/anthropic/chat_tool_choice_required_in.json

@@ -0,0 +1,9 @@
+{
+  "model": "claude-opus-5",
+  "messages": [{"role": "user", "content": "Do something"}],
+  "max_tokens": 100,
+  "tools": [
+    {"type": "function", "function": {"name": "act", "description": "acts", "parameters": {"type": "object", "properties": {}}}}
+  ],
+  "tool_choice": "required"
+}

testdata/anthropic/chat_tool_choice_required_want.json

@@ -0,0 +1,9 @@
+{
+  "model": "claude-opus-5",
+  "messages": [{"role": "user", "content": "Do something"}],
+  "max_tokens": 100,
+  "tools": [
+    {"name": "act", "description": "acts", "input_schema": {"type": "object", "properties": {}}}
+  ],
+  "tool_choice": {"type": "any"}
+}

testdata/anthropic/chat_tool_choice_specific_in.json

@@ -0,0 +1,9 @@
+{
+  "model": "claude-opus-5",
+  "messages": [{"role": "user", "content": "Do something"}],
+  "max_tokens": 100,
+  "tools": [
+    {"type": "function", "function": {"name": "act", "description": "acts", "parameters": {"type": "object", "properties": {}}}}
+  ],
+  "tool_choice": {"type": "function", "function": {"name": "act"}}
+}

testdata/anthropic/chat_tool_choice_specific_want.json

@@ -0,0 +1,9 @@
+{
+  "model": "claude-opus-5",
+  "messages": [{"role": "user", "content": "Do something"}],
+  "max_tokens": 100,
+  "tools": [
+    {"name": "act", "description": "acts", "input_schema": {"type": "object", "properties": {}}}
+  ],
+  "tool_choice": {"type": "tool", "name": "act"}
+}

testdata/anthropic/chat_unsupported_logit_bias_in.json

@@ -0,0 +1,5 @@
+{
+  "model": "claude-opus-5",
+  "messages": [{"role": "user", "content": "hi"}],
+  "logit_bias": {"50256": -100}
+}

testdata/anthropic/chat_unsupported_logit_bias_wanterr.json

@@ -0,0 +1 @@
+{"errContains": "logit_bias is not supported for anthropic models", "notSupported": false}

testdata/anthropic/chat_unsupported_logprobs_in.json

@@ -0,0 +1,5 @@
+{
+  "model": "claude-opus-5",
+  "messages": [{"role": "user", "content": "hi"}],
+  "logprobs": true
+}

testdata/anthropic/chat_unsupported_logprobs_wanterr.json

@@ -0,0 +1 @@
+{"errContains": "logprobs is not supported for anthropic models", "notSupported": false}

testdata/anthropic/chat_unsupported_n_in.json

@@ -0,0 +1,5 @@
+{
+  "model": "claude-opus-5",
+  "messages": [{"role": "user", "content": "hi"}],
+  "n": 2
+}

testdata/anthropic/chat_unsupported_n_wanterr.json

@@ -0,0 +1 @@
+{"errContains": "n is not supported for anthropic models", "notSupported": false}

testdata/anthropic/resp_maxtokens_in.json

@@ -0,0 +1,12 @@
+{
+  "id": "msg_01TRUNC",
+  "type": "message",
+  "role": "assistant",
+  "content": [
+    {"type": "text", "text": "This response was cut"}
+  ],
+  "model": "claude-opus-5",
+  "stop_reason": "max_tokens",
+  "stop_sequence": null,
+  "usage": {"input_tokens": 5, "output_tokens": 100}
+}

testdata/anthropic/resp_maxtokens_want.json

@@ -0,0 +1,14 @@
+{
+  "id": "chatcmpl-msg_01TRUNC",
+  "object": "chat.completion",
+  "created": 1734000000,
+  "model": "gw-anthropic-model",
+  "choices": [
+    {
+      "index": 0,
+      "message": {"role": "assistant", "content": "This response was cut"},
+      "finish_reason": "length"
+    }
+  ],
+  "usage": {"prompt_tokens": 5, "completion_tokens": 100, "total_tokens": 105}
+}

testdata/anthropic/resp_text_in.json

@@ -0,0 +1,12 @@
+{
+  "id": "msg_01XYZ",
+  "type": "message",
+  "role": "assistant",
+  "content": [
+    {"type": "text", "text": "The capital of France is Paris."}
+  ],
+  "model": "claude-opus-5",
+  "stop_reason": "end_turn",
+  "stop_sequence": null,
+  "usage": {"input_tokens": 12, "output_tokens": 8}
+}

testdata/anthropic/resp_text_want.json

@@ -0,0 +1,14 @@
+{
+  "id": "chatcmpl-msg_01XYZ",
+  "object": "chat.completion",
+  "created": 1734000000,
+  "model": "gw-anthropic-model",
+  "choices": [
+    {
+      "index": 0,
+      "message": {"role": "assistant", "content": "The capital of France is Paris."},
+      "finish_reason": "stop"
+    }
+  ],
+  "usage": {"prompt_tokens": 12, "completion_tokens": 8, "total_tokens": 20}
+}

testdata/anthropic/resp_tooluse_in.json

@@ -0,0 +1,13 @@
+{
+  "id": "msg_01ABC",
+  "type": "message",
+  "role": "assistant",
+  "content": [
+    {"type": "text", "text": "Let me check the weather."},
+    {"type": "tool_use", "id": "toolu_01DEF", "name": "get_weather", "input": {"location": "Paris"}}
+  ],
+  "model": "claude-opus-5",
+  "stop_reason": "tool_use",
+  "stop_sequence": null,
+  "usage": {"input_tokens": 20, "output_tokens": 15}
+}

testdata/anthropic/resp_tooluse_want.json

@@ -0,0 +1,20 @@
+{
+  "id": "chatcmpl-msg_01ABC",
+  "object": "chat.completion",
+  "created": 1734000000,
+  "model": "gw-anthropic-model",
+  "choices": [
+    {
+      "index": 0,
+      "message": {
+        "role": "assistant",
+        "content": "Let me check the weather.",
+        "tool_calls": [
+          {"id": "toolu_01DEF", "type": "function", "function": {"name": "get_weather", "arguments": "{\"location\":\"Paris\"}"}}
+        ]
+      },
+      "finish_reason": "tool_calls"
+    }
+  ],
+  "usage": {"prompt_tokens": 20, "completion_tokens": 15, "total_tokens": 35}
+}

testdata/anthropic/stream_text.sse

@@ -0,0 +1,24 @@
+event: message_start
+data: {"type":"message_start","message":{"id":"msg_01STREAM","type":"message","role":"assistant","content":[],"model":"claude-opus-5","stop_reason":null,"stop_sequence":null,"usage":{"input_tokens":10,"output_tokens":1}}}
+
+event: content_block_start
+data: {"type":"content_block_start","index":0,"content_block":{"type":"text","text":""}}
+
+event: ping
+data: {"type":"ping"}
+
+event: content_block_delta
+data: {"type":"content_block_delta","index":0,"delta":{"type":"text_delta","text":"Hello"}}
+
+event: content_block_delta
+data: {"type":"content_block_delta","index":0,"delta":{"type":"text_delta","text":" world"}}
+
+event: content_block_stop
+data: {"type":"content_block_stop","index":0}
+
+event: message_delta
+data: {"type":"message_delta","delta":{"stop_reason":"end_turn","stop_sequence":null},"usage":{"output_tokens":5}}
+
+event: message_stop
+data: {"type":"message_stop"}
+

testdata/anthropic/stream_text_usage.json

@@ -0,0 +1 @@
+{"prompt": 10, "completion": 5}

testdata/anthropic/stream_text_want.json

@@ -0,0 +1,6 @@
+[
+  {"id":"chatcmpl-msg_01STREAM","object":"chat.completion.chunk","created":1734000000,"model":"gw-anthropic-model","choices":[{"index":0,"delta":{"role":"assistant"},"finish_reason":null}]},
+  {"id":"chatcmpl-msg_01STREAM","object":"chat.completion.chunk","created":1734000000,"model":"gw-anthropic-model","choices":[{"index":0,"delta":{"content":"Hello"},"finish_reason":null}]},
+  {"id":"chatcmpl-msg_01STREAM","object":"chat.completion.chunk","created":1734000000,"model":"gw-anthropic-model","choices":[{"index":0,"delta":{"content":" world"},"finish_reason":null}]},
+  {"id":"chatcmpl-msg_01STREAM","object":"chat.completion.chunk","created":1734000000,"model":"gw-anthropic-model","choices":[{"index":0,"delta":{},"finish_reason":"stop"}]}
+]

testdata/anthropic/stream_tooluse.sse

@@ -0,0 +1,21 @@
+event: message_start
+data: {"type":"message_start","message":{"id":"msg_01TOOL","type":"message","role":"assistant","content":[],"model":"claude-opus-5","stop_reason":null,"stop_sequence":null,"usage":{"input_tokens":15,"output_tokens":1}}}
+
+event: content_block_start
+data: {"type":"content_block_start","index":0,"content_block":{"type":"tool_use","id":"toolu_01XYZ","name":"get_weather","input":{}}}
+
+event: content_block_delta
+data: {"type":"content_block_delta","index":0,"delta":{"type":"input_json_delta","partial_json":"{\"location\": \""}}
+
+event: content_block_delta
+data: {"type":"content_block_delta","index":0,"delta":{"type":"input_json_delta","partial_json":"Paris\"}"}}
+
+event: content_block_stop
+data: {"type":"content_block_stop","index":0}
+
+event: message_delta
+data: {"type":"message_delta","delta":{"stop_reason":"tool_use","stop_sequence":null},"usage":{"output_tokens":7}}
+
+event: message_stop
+data: {"type":"message_stop"}
+

testdata/anthropic/stream_tooluse_usage.json

@@ -0,0 +1 @@
+{"prompt": 15, "completion": 7}

testdata/anthropic/stream_tooluse_want.json

@@ -0,0 +1,7 @@
+[
+  {"id":"chatcmpl-msg_01TOOL","object":"chat.completion.chunk","created":1734000000,"model":"gw-anthropic-model","choices":[{"index":0,"delta":{"role":"assistant"},"finish_reason":null}]},
+  {"id":"chatcmpl-msg_01TOOL","object":"chat.completion.chunk","created":1734000000,"model":"gw-anthropic-model","choices":[{"index":0,"delta":{"tool_calls":[{"index":0,"id":"toolu_01XYZ","type":"function","function":{"name":"get_weather","arguments":""}}]},"finish_reason":null}]},
+  {"id":"chatcmpl-msg_01TOOL","object":"chat.completion.chunk","created":1734000000,"model":"gw-anthropic-model","choices":[{"index":0,"delta":{"tool_calls":[{"index":0,"function":{"arguments":"{\"location\": \""}}]},"finish_reason":null}]},
+  {"id":"chatcmpl-msg_01TOOL","object":"chat.completion.chunk","created":1734000000,"model":"gw-anthropic-model","choices":[{"index":0,"delta":{"tool_calls":[{"index":0,"function":{"arguments":"Paris\"}"}}]},"finish_reason":null}]},
+  {"id":"chatcmpl-msg_01TOOL","object":"chat.completion.chunk","created":1734000000,"model":"gw-anthropic-model","choices":[{"index":0,"delta":{},"finish_reason":"tool_calls"}]}
+]