fix(ai): replay Claude assistant turns verbatim so thinking blocks keep their signature

Claude Sonnet 5.5 는 응답에 thinking 블록을 포함한다(adaptive thinking, 비활성화 불가).
어댑터가 어시스턴트 턴을 자체 구조체로 재직렬화하면서 thinking 본문과 signature 를
떨어뜨려 두 번째 턴에서 400(thinking.thinking: Field required)이 났다(2026-10-15 C-G1
실측). 응답 content 를 json.RawMessage 로 보관해 그대로 재전송한다. 테스트에 thinking
블록 재전송 검증을 추가.

ai/internal/anthropic/client_test.go

@@ -24,7 +24,7 @@ func TestTranslateTurns(t *testing.T) {
 		bodies = append(bodies, b)
 		switch len(bodies) {
 		case 1:
-			w.Write([]byte(`{"id":"msg_1","type":"message","model":"claude-sonnet-5-5","stop_reason":"tool_use","content":[{"type":"text","text":"checking"},{"type":"tool_use","id":"toolu_1","name":"get_probe_status","input":{}}],"usage":{"input_tokens":100,"output_tokens":20,"cache_creation_input_tokens":5,"cache_read_input_tokens":0}}`))
+			w.Write([]byte(`{"id":"msg_1","type":"message","model":"claude-sonnet-5-5","stop_reason":"tool_use","content":[{"type":"thinking","thinking":"let me probe","signature":"sig-abc"},{"type":"text","text":"checking"},{"type":"tool_use","id":"toolu_1","name":"get_probe_status","input":{}}],"usage":{"input_tokens":100,"output_tokens":20,"cache_creation_input_tokens":5,"cache_read_input_tokens":0}}`))
 		default:
 			w.Write([]byte(`{"id":"msg_2","type":"message","model":"claude-sonnet-5-5","stop_reason":"tool_use","content":[{"type":"tool_use","id":"toolu_2","name":"submit_diagnosis","input":{"root_cause_label":"x","confidence":0.9}}],"usage":{"input_tokens":200,"output_tokens":30}}`))
 		}
@@ -75,6 +75,10 @@ func TestTranslateTurns(t *testing.T) {
 	if as["role"] != "assistant" || !strings.Contains(mustJSON(as["content"]), `"tool_use"`) {
 		t.Fatalf("assistant turn not replayed: %v", as)
 	}
+	// the thinking block must come back verbatim with its text and signature (Claude 5.5 rejects a stripped one)
+	if ac := mustJSON(as["content"]); !strings.Contains(ac, `"thinking":"let me probe"`) || !strings.Contains(ac, `"signature":"sig-abc"`) {
+		t.Fatalf("thinking block not replayed verbatim: %s", ac)
+	}
 	last := msgs[2].(map[string]any)["content"].([]any)
 	if tr := last[0].(map[string]any); tr["type"] != "tool_result" || tr["tool_use_id"] != "toolu_1" || tr["content"] != `{"alb":1}` {
 		t.Fatalf("tool_result: %v", tr)