fix(backend): 改进 thinking/tool block 签名处理和重试策略

主要改动：
- request_transformer: thinking block 缺少签名时降级为文本而非丢弃，保留内容并在上层禁用 thinking mode
- antigravity_gateway_service: 新增两阶段降级策略，先处理 thinking blocks，如仍失败且涉及 tool 签名错误则进一步降级 tool blocks
- gateway_request: 新增 FilterSignatureSensitiveBlocksForRetry 函数，支持将 tool_use/tool_result 降级为文本
- gateway_request: 改进 FilterThinkingBlocksForRetry，禁用顶层 thinking 配置以避免结构约束冲突
- gateway_service: 实现保守的两阶段重试逻辑，优先保留内容，仅在必要时降级工具调用
- 新增 antigravity_gateway_service_test.go 测试签名块剥离逻辑
- 更新相关测试用例以验证降级行为

此修复解决了跨平台/账户切换时历史消息签名失效导致的请求失败问题。

backend/internal/pkg/antigravity/request_transformer.go

@@ -22,7 +22,7 @@ func TransformClaudeToGemini(claudeReq *ClaudeRequest, projectID, mappedModel st
 	allowDummyThought := strings.HasPrefix(mappedModel, "gemini-")
 
 	// 1. 构建 contents
-	contents, err := buildContents(claudeReq.Messages, toolIDToName, isThinkingEnabled, allowDummyThought)
+	contents, strippedThinking, err := buildContents(claudeReq.Messages, toolIDToName, isThinkingEnabled, allowDummyThought)
 	if err != nil {
 		return nil, fmt.Errorf("build contents: %w", err)
 	}
@@ -31,7 +31,15 @@ func TransformClaudeToGemini(claudeReq *ClaudeRequest, projectID, mappedModel st
 	systemInstruction := buildSystemInstruction(claudeReq.System, claudeReq.Model)
 
 	// 3. 构建 generationConfig
-	generationConfig := buildGenerationConfig(claudeReq)
+	reqForConfig := claudeReq
+	if strippedThinking {
+		// If we had to downgrade thinking blocks to plain text due to missing/invalid signatures,
+		// disable upstream thinking mode to avoid signature/structure validation errors.
+		reqCopy := *claudeReq
+		reqCopy.Thinking = nil
+		reqForConfig = &reqCopy
+	}
+	generationConfig := buildGenerationConfig(reqForConfig)
 
 	// 4. 构建 tools
 	tools := buildTools(claudeReq.Tools)
@@ -120,8 +128,9 @@ func buildSystemInstruction(system json.RawMessage, modelName string) *GeminiCon
 }
 
 // buildContents 构建 contents
-func buildContents(messages []ClaudeMessage, toolIDToName map[string]string, isThinkingEnabled, allowDummyThought bool) ([]GeminiContent, error) {
+func buildContents(messages []ClaudeMessage, toolIDToName map[string]string, isThinkingEnabled, allowDummyThought bool) ([]GeminiContent, bool, error) {
 	var contents []GeminiContent
+	strippedThinking := false
 
 	for i, msg := range messages {
 		role := msg.Role
@@ -129,9 +138,12 @@ func buildContents(messages []ClaudeMessage, toolIDToName map[string]string, isT
 			role = "model"
 		}
 
-		parts, err := buildParts(msg.Content, toolIDToName, allowDummyThought)
+		parts, strippedThisMsg, err := buildParts(msg.Content, toolIDToName, allowDummyThought)
 		if err != nil {
-			return nil, fmt.Errorf("build parts for message %d: %w", i, err)
+			return nil, false, fmt.Errorf("build parts for message %d: %w", i, err)
+		}
+		if strippedThisMsg {
+			strippedThinking = true
 		}
 
 		// 只有 Gemini 模型支持 dummy thinking block workaround
@@ -165,7 +177,7 @@ func buildContents(messages []ClaudeMessage, toolIDToName map[string]string, isT
 		})
 	}
 
-	return contents, nil
+	return contents, strippedThinking, nil
 }
 
 // dummyThoughtSignature 用于跳过 Gemini 3 thought_signature 验证
@@ -174,8 +186,9 @@ const dummyThoughtSignature = "skip_thought_signature_validator"
 
 // buildParts 构建消息的 parts
 // allowDummyThought: 只有 Gemini 模型支持 dummy thought signature
-func buildParts(content json.RawMessage, toolIDToName map[string]string, allowDummyThought bool) ([]GeminiPart, error) {
+func buildParts(content json.RawMessage, toolIDToName map[string]string, allowDummyThought bool) ([]GeminiPart, bool, error) {
 	var parts []GeminiPart
+	strippedThinking := false
 
 	// 尝试解析为字符串
 	var textContent string
@@ -183,13 +196,13 @@ func buildParts(content json.RawMessage, toolIDToName map[string]string, allowDu
 		if textContent != "(no content)" && strings.TrimSpace(textContent) != "" {
 			parts = append(parts, GeminiPart{Text: strings.TrimSpace(textContent)})
 		}
-		return parts, nil
+		return parts, false, nil
 	}
 
 	// 解析为内容块数组
 	var blocks []ContentBlock
 	if err := json.Unmarshal(content, &blocks); err != nil {
-		return nil, fmt.Errorf("parse content blocks: %w", err)
+		return nil, false, fmt.Errorf("parse content blocks: %w", err)
 	}
 
 	for _, block := range blocks {
@@ -208,8 +221,11 @@ func buildParts(content json.RawMessage, toolIDToName map[string]string, allowDu
 			if block.Signature != "" {
 				part.ThoughtSignature = block.Signature
 			} else if !allowDummyThought {
-				// Claude 模型需要有效 signature，跳过无 signature 的 thinking block
-				log.Printf("Warning: skipping thinking block without signature for Claude model")
+				// Claude 模型需要有效 signature；在缺失时降级为普通文本，并在上层禁用 thinking mode。
+				if strings.TrimSpace(block.Thinking) != "" {
+					parts = append(parts, GeminiPart{Text: block.Thinking})
+				}
+				strippedThinking = true
 				continue
 			} else {
 				// Gemini 模型使用 dummy signature
@@ -276,7 +292,7 @@ func buildParts(content json.RawMessage, toolIDToName map[string]string, allowDu
 		}
 	}
 
-	return parts, nil
+	return parts, strippedThinking, nil
 }
 
 // parseToolResultContent 解析 tool_result 的 content

backend/internal/pkg/antigravity/request_transformer_test.go

@@ -15,15 +15,15 @@ func TestBuildParts_ThinkingBlockWithoutSignature(t *testing.T) {
 		description       string
 	}{
 		{
-			name: "Claude model - drop thinking without signature",
+			name: "Claude model - downgrade thinking to text without signature",
 			content: `[
 				{"type": "text", "text": "Hello"},
 				{"type": "thinking", "thinking": "Let me think...", "signature": ""},
 				{"type": "text", "text": "World"}
 			]`,
 			allowDummyThought: false,
-			expectedParts:     2, // thinking 内容被丢弃
-			description:       "Claude模型应丢弃无signature的thinking block内容",
+			expectedParts:     3, // thinking 内容降级为普通 text part
+			description:       "Claude模型缺少signature时应将thinking降级为text，并在上层禁用thinking mode",
 		},
 		{
 			name: "Claude model - preserve thinking block with signature",
@@ -52,7 +52,7 @@ func TestBuildParts_ThinkingBlockWithoutSignature(t *testing.T) {
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {
 			toolIDToName := make(map[string]string)
-			parts, err := buildParts(json.RawMessage(tt.content), toolIDToName, tt.allowDummyThought)
+			parts, _, err := buildParts(json.RawMessage(tt.content), toolIDToName, tt.allowDummyThought)
 
 			if err != nil {
 				t.Fatalf("buildParts() error = %v", err)
@@ -71,6 +71,17 @@ func TestBuildParts_ThinkingBlockWithoutSignature(t *testing.T) {
 					t.Fatalf("expected thought part with signature sig_real_123, got thought=%v signature=%q",
 						parts[1].Thought, parts[1].ThoughtSignature)
 				}
+			case "Claude model - downgrade thinking to text without signature":
+				if len(parts) != 3 {
+					t.Fatalf("expected 3 parts, got %d", len(parts))
+				}
+				if parts[1].Thought {
+					t.Fatalf("expected downgraded text part, got thought=%v signature=%q",
+						parts[1].Thought, parts[1].ThoughtSignature)
+				}
+				if parts[1].Text != "Let me think..." {
+					t.Fatalf("expected downgraded text %q, got %q", "Let me think...", parts[1].Text)
+				}
 			case "Gemini model - use dummy signature":
 				if len(parts) != 3 {
 					t.Fatalf("expected 3 parts, got %d", len(parts))
@@ -91,7 +102,7 @@ func TestBuildParts_ToolUseSignatureHandling(t *testing.T) {
 
 	t.Run("Gemini uses dummy tool_use signature", func(t *testing.T) {
 		toolIDToName := make(map[string]string)
-		parts, err := buildParts(json.RawMessage(content), toolIDToName, true)
+		parts, _, err := buildParts(json.RawMessage(content), toolIDToName, true)
 		if err != nil {
 			t.Fatalf("buildParts() error = %v", err)
 		}
@@ -105,7 +116,7 @@ func TestBuildParts_ToolUseSignatureHandling(t *testing.T) {
 
 	t.Run("Claude model - preserve valid signature for tool_use", func(t *testing.T) {
 		toolIDToName := make(map[string]string)
-		parts, err := buildParts(json.RawMessage(content), toolIDToName, false)
+		parts, _, err := buildParts(json.RawMessage(content), toolIDToName, false)
 		if err != nil {
 			t.Fatalf("buildParts() error = %v", err)
 		}