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.go

@@ -135,11 +135,11 @@ type request struct {
 }
 
 type response struct {
-	ID         string  `json:"id"`
-	Type       string  `json:"type"`
-	Model      string  `json:"model"`
-	StopReason string  `json:"stop_reason"`
-	Content    []block `json:"content"`
+	ID         string          `json:"id"`
+	Type       string          `json:"type"`
+	Model      string          `json:"model"`
+	StopReason string          `json:"stop_reason"`
+	Content    json.RawMessage `json:"content"` // kept verbatim: thinking blocks (with their signature) must be replayed unchanged
 	Usage      struct {
 		InputTokens              int `json:"input_tokens"`
 		OutputTokens             int `json:"output_tokens"`
@@ -275,8 +275,10 @@ func (c *Client) CreateResponse(ctx context.Context, req openai.Request) (*opena
 		out.ID = fmt.Sprintf("msg_local_%d", c.seq)
 		c.mu.Unlock()
 	}
+	var blocks []block
+	_ = json.Unmarshal(r.Content, &blocks)
 	var text strings.Builder
-	for _, bl := range r.Content {
+	for _, bl := range blocks {
 		switch bl.Type {
 		case "text":
 			text.WriteString(bl.Text)
@@ -296,7 +298,9 @@ func (c *Client) CreateResponse(ctx context.Context, req openai.Request) (*opena
 	out.Usage.OutputTokens = r.Usage.OutputTokens
 	out.Usage.TotalTokens = out.Usage.InputTokens + out.Usage.OutputTokens
 
-	// remember the conversation for the next turn (assistant content kept verbatim so tool_use ids line up)
+	// remember the conversation for the next turn: the assistant content is replayed as the raw JSON the API returned,
+	// so tool_use ids and thinking blocks (adaptive thinking is always on for Claude 5.5; a thinking block replayed
+	// without its text/signature is rejected with "thinking.thinking: Field required") stay intact
 	full := append(append([]message{}, msgs...), message{Role: "assistant", Content: r.Content})
 	c.mu.Lock()
 	c.conv[out.ID] = conversation{system: body.System, msgs: full}