fix(anthropic): merge parallel tool results, ordinal stream tool indices, array system content, strict stream decode

Review round 1 fixes (1 Critical + 5 Important) on the anthropic
adapter/translator.

- CRITICAL: consecutive role:"tool" messages each became their own
  Anthropic user turn. Anthropic requires alternating user/assistant
  turns and 400s on two consecutive same-role messages; parallel tool
  calls (one assistant message with N tool_calls followed by N
  role:"tool" messages) are the standard agentic transcript shape.
  anthropicRequestFromOpenAI now accumulates consecutive tool messages
  into one Anthropic user turn carrying all of their tool_result
  blocks (anthropicToolResultBlock replaces anthropicToolResultMessage).
  Added chat_parallel_tool_results fixture.
- Streaming tool_calls[].index passed Anthropic's raw content-block
  index through, so a tool_use block after a text block emitted
  index:1 instead of the required contiguous 0-based ordinal.
  anthropicStreamState now tracks a per-stream tool-call ordinal
  counter plus a content-block-index -> ordinal map, populated on
  content_block_start and consulted on input_json_delta. Added the
  stream_text_then_tool fixture (text block at index 0, tool_use at
  index 1) proving the emitted index is 0.
- A system message with array-form content ([{"type":"text",...}])
  was silently dropped (only a string content was handled). Added
  systemTextFromContentParts: text parts join with "\n"; a non-text
  part returns a *translateError instead of being dropped. Added
  chat_system_array_content (success) and
  chat_system_array_content_error fixtures.
- Malformed JSON on a recognized stream event (message_start,
  content_block_start, content_block_delta, message_delta) was
  swallowed as (nil, nil), which could silently corrupt a client's
  reassembled tool-call arguments. All four now return an error
  wrapping errUpstream; forwardStream already propagates that and
  withholds the terminal [DONE]. The unrecognized-event-type and
  error-event branches are unchanged. Added
  TestAnthropicStreamState_MalformedEventReturnsError.
- providerAdapter.injectAuth's doc comment claimed a universal
  empty-key no-op; updated to state the per-type divergence
  (openai-type: keyless is a no-op; anthropic-type: the constructor
  rejects an empty key, so injectAuth is never called with one).
- Added case-insensitive-Content-Type coverage for both adapters'
  streaming-vs-JSON gate (previously untested): one subtest each in
  provider_openai_test.go / provider_anthropic_test.go using a fake
  upstream that sends "Text/Event-Stream; charset=utf-8", asserting
  the streaming path (not the JSON fallback) is taken.

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

testdata/anthropic/chat_parallel_tool_results_in.json

@@ -0,0 +1,13 @@
+{
+  "model": "claude-opus-5",
+  "messages": [
+    {"role": "user", "content": "What's the weather in Paris and Tokyo?"},
+    {"role": "assistant", "content": null, "tool_calls": [
+      {"id": "call_1", "type": "function", "function": {"name": "get_weather", "arguments": "{\"location\":\"Paris\"}"}},
+      {"id": "call_2", "type": "function", "function": {"name": "get_weather", "arguments": "{\"location\":\"Tokyo\"}"}}
+    ]},
+    {"role": "tool", "tool_call_id": "call_1", "content": "72F and sunny"},
+    {"role": "tool", "tool_call_id": "call_2", "content": "50F and rainy"}
+  ],
+  "max_tokens": 200
+}

testdata/anthropic/chat_parallel_tool_results_want.json

@@ -0,0 +1,15 @@
+{
+  "model": "claude-opus-5",
+  "messages": [
+    {"role": "user", "content": "What's the weather in Paris and Tokyo?"},
+    {"role": "assistant", "content": [
+      {"type": "tool_use", "id": "call_1", "name": "get_weather", "input": {"location": "Paris"}},
+      {"type": "tool_use", "id": "call_2", "name": "get_weather", "input": {"location": "Tokyo"}}
+    ]},
+    {"role": "user", "content": [
+      {"type": "tool_result", "tool_use_id": "call_1", "content": "72F and sunny"},
+      {"type": "tool_result", "tool_use_id": "call_2", "content": "50F and rainy"}
+    ]}
+  ],
+  "max_tokens": 200
+}

testdata/anthropic/chat_system_array_content_error_in.json

@@ -0,0 +1,11 @@
+{
+  "model": "claude-opus-5",
+  "messages": [
+    {"role": "system", "content": [
+      {"type": "text", "text": "You are a helpful assistant."},
+      {"type": "image_url", "image_url": {"url": "data:image/png;base64,AAAA"}}
+    ]},
+    {"role": "user", "content": "Hi"}
+  ],
+  "max_tokens": 100
+}

testdata/anthropic/chat_system_array_content_error_wanterr.json

@@ -0,0 +1 @@
+{"errContains": "system message content part type \"image_url\" is not supported for anthropic models", "notSupported": false}

testdata/anthropic/chat_system_array_content_in.json

@@ -0,0 +1,11 @@
+{
+  "model": "claude-opus-5",
+  "messages": [
+    {"role": "system", "content": [
+      {"type": "text", "text": "You are a helpful assistant."},
+      {"type": "text", "text": "Always be concise."}
+    ]},
+    {"role": "user", "content": "Hi"}
+  ],
+  "max_tokens": 100
+}

testdata/anthropic/chat_system_array_content_want.json

@@ -0,0 +1,8 @@
+{
+  "model": "claude-opus-5",
+  "system": "You are a helpful assistant.\nAlways be concise.",
+  "messages": [
+    {"role": "user", "content": "Hi"}
+  ],
+  "max_tokens": 100
+}

testdata/anthropic/stream_text_then_tool.sse

@@ -0,0 +1,27 @@
+event: message_start
+data: {"type":"message_start","message":{"id":"msg_01MIX","type":"message","role":"assistant","content":[],"model":"claude-opus-5","stop_reason":null,"stop_sequence":null,"usage":{"input_tokens":8,"output_tokens":1}}}
+
+event: content_block_start
+data: {"type":"content_block_start","index":0,"content_block":{"type":"text","text":""}}
+
+event: content_block_delta
+data: {"type":"content_block_delta","index":0,"delta":{"type":"text_delta","text":"Let me check."}}
+
+event: content_block_stop
+data: {"type":"content_block_stop","index":0}
+
+event: content_block_start
+data: {"type":"content_block_start","index":1,"content_block":{"type":"tool_use","id":"toolu_MIX1","name":"get_weather","input":{}}}
+
+event: content_block_delta
+data: {"type":"content_block_delta","index":1,"delta":{"type":"input_json_delta","partial_json":"{\"location\":\"Paris\"}"}}
+
+event: content_block_stop
+data: {"type":"content_block_stop","index":1}
+
+event: message_delta
+data: {"type":"message_delta","delta":{"stop_reason":"tool_use","stop_sequence":null},"usage":{"output_tokens":6}}
+
+event: message_stop
+data: {"type":"message_stop"}
+

testdata/anthropic/stream_text_then_tool_usage.json

@@ -0,0 +1 @@
+{"prompt": 8, "completion": 6}

testdata/anthropic/stream_text_then_tool_want.json

@@ -0,0 +1,7 @@
+[
+  {"id":"chatcmpl-msg_01MIX","object":"chat.completion.chunk","created":1734000000,"model":"gw-anthropic-model","choices":[{"index":0,"delta":{"role":"assistant"},"finish_reason":null}]},
+  {"id":"chatcmpl-msg_01MIX","object":"chat.completion.chunk","created":1734000000,"model":"gw-anthropic-model","choices":[{"index":0,"delta":{"content":"Let me check."},"finish_reason":null}]},
+  {"id":"chatcmpl-msg_01MIX","object":"chat.completion.chunk","created":1734000000,"model":"gw-anthropic-model","choices":[{"index":0,"delta":{"tool_calls":[{"index":0,"id":"toolu_MIX1","type":"function","function":{"name":"get_weather","arguments":""}}]},"finish_reason":null}]},
+  {"id":"chatcmpl-msg_01MIX","object":"chat.completion.chunk","created":1734000000,"model":"gw-anthropic-model","choices":[{"index":0,"delta":{"tool_calls":[{"index":0,"function":{"arguments":"{\"location\":\"Paris\"}"}}]},"finish_reason":null}]},
+  {"id":"chatcmpl-msg_01MIX","object":"chat.completion.chunk","created":1734000000,"model":"gw-anthropic-model","choices":[{"index":0,"delta":{},"finish_reason":"tool_calls"}]}
+]