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.

provider_openai_test.go

@@ -557,13 +557,32 @@ func TestBuildAdapters(t *testing.T) {
 		}
 	})
 
-	t.Run("edge: anthropic type not yet implemented", func(t *testing.T) {
+	t.Run("good: anthropic with an API key builds successfully", func(t *testing.T) {
 		cfg := &Config{Providers: map[string]*ProviderConfig{
 			"p1": {Type: "anthropic", APIKey: "sk-test"},
 		}}
-		_, err := buildAdapters(cfg)
-		if err == nil {
-			t.Fatalf("buildAdapters: want placeholder error for anthropic, got nil")
+		adapters, err := buildAdapters(cfg)
+		if err != nil {
+			t.Fatalf("buildAdapters: %v", err)
+		}
+		a, ok := adapters["p1"]
+		if !ok {
+			t.Fatalf("adapters[%q] missing", "p1")
+		}
+		if a.typeName() != "anthropic" {
+			t.Errorf("typeName() = %q, want %q", a.typeName(), "anthropic")
+		}
+		if a.base() != defaultBaseAnthropic {
+			t.Errorf("base() = %q, want %q", a.base(), defaultBaseAnthropic)
+		}
+	})
+
+	t.Run("bad: anthropic without an API key is a constructor error", func(t *testing.T) {
+		cfg := &Config{Providers: map[string]*ProviderConfig{
+			"p1": {Type: "anthropic"},
+		}}
+		if _, err := buildAdapters(cfg); err == nil {
+			t.Fatalf("buildAdapters: want error for keyless anthropic provider, got nil")
 		}
 	})