fix: retry on "thinking block must contain thinking" upstream error

Some clients reuse assistant history from other models when switching to
claude with extended thinking enabled. If a prior thinking block lacks the
thinking text field, upstream returns:
  messages.X.content.Y.thinking: each thinking block must contain thinking

Add this pattern to isThinkingBlockSignatureError so the existing
FilterThinkingBlocksForRetry retry path triggers and rewrites/drops the
offending blocks.

backend/internal/service/gateway_request_test.go

@@ -379,6 +379,37 @@ func TestFilterThinkingBlocksForRetry_RemovesRedactedThinkingAndKeepsValidConten
 	require.Equal(t, "Visible", content0["text"])
 }
 
+func TestFilterThinkingBlocksForRetry_DropsThinkingBlockWithEmptyContent(t *testing.T) {
+	// 跨模型场景：其他模型回过的 assistant 历史里携带了 type=thinking 但 thinking 字段为空，
+	// 喂给开启 extended thinking 的 claude 时上游会报：
+	//   "messages.1.content.0.thinking: each thinking block must contain thinking"
+	// 重试应当把空 thinking 块丢弃，并保留其它有效内容。
+	input := []byte(`{
+		"thinking":{"type":"enabled","budget_tokens":1024},
+		"messages":[
+			{"role":"user","content":[{"type":"text","text":"Hi"}]},
+			{"role":"assistant","content":[
+				{"type":"thinking","thinking":"","signature":"sig"},
+				{"type":"text","text":"Answer"}
+			]}
+		]
+	}`)
+
+	out := FilterThinkingBlocksForRetry(input)
+
+	var req map[string]any
+	require.NoError(t, json.Unmarshal(out, &req))
+	_, hasThinking := req["thinking"]
+	require.False(t, hasThinking, "top-level thinking should be removed")
+
+	msgs := req["messages"].([]any)
+	assistant := msgs[1].(map[string]any)
+	content := assistant["content"].([]any)
+	require.Len(t, content, 1, "empty thinking block should be dropped, only text remains")
+	require.Equal(t, "text", content[0].(map[string]any)["type"])
+	require.Equal(t, "Answer", content[0].(map[string]any)["text"])
+}
+
 func TestFilterThinkingBlocksForRetry_EmptyContentGetsPlaceholder(t *testing.T) {
 	input := []byte(`{
 		"thinking":{"type":"enabled"},

backend/internal/service/gateway_service.go

@@ -6765,6 +6765,15 @@ func (s *GatewayService) isThinkingBlockSignatureError(respBody []byte) bool {
 		return true
 	}
 
+	// 检测 thinking block 缺少 thinking 字段的错误（跨模型切换时常见：
+	// 其他模型回过的 assistant 历史里有 type=thinking 但没有 thinking 文本，
+	// 喂给开启 extended thinking 的 claude 时会被拒）
+	// 例如: "messages.1.content.0.thinking: each thinking block must contain thinking"
+	if strings.Contains(msg, "thinking block must contain") {
+		logger.LegacyPrintf("service.gateway", "[SignatureCheck] Detected thinking block missing content error")
+		return true
+	}
+
 	return false
 }