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_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)
 		}