fix(antigravity): 修复 Gemini 3 thought_signature 和 schema 验证问题

- 添加 dummyThoughtSignature 常量，在 thinking 模式下为无 signature 的 tool_use 自动添加
- 增强 cleanJSONSchema：过滤 required 中不存在的属性，确保 type/properties 字段存在
- 扩展 excludedSchemaKeys：增加 $id, $ref, strict, const, examples 等不支持的字段
- 修复 429 重试逻辑：仅在所有重试失败后才标记账户为 rate_limited
- 添加 e2e 集成测试：TestClaudeMessagesWithThinkingAndTools

backend/internal/service/antigravity_gateway_service.go

@@ -271,14 +271,15 @@ func (s *AntigravityGatewayService) Forward(ctx context.Context, c *gin.Context,
 			respBody, _ := io.ReadAll(io.LimitReader(resp.Body, 2<<20))
 			_ = resp.Body.Close()
 
-			if resp.StatusCode == 429 {
-				s.handleUpstreamError(ctx, account, resp.StatusCode, resp.Header, respBody)
-			}
 			if attempt < antigravityMaxRetries {
 				log.Printf("Antigravity account %d: upstream status %d, retry %d/%d", account.ID, resp.StatusCode, attempt, antigravityMaxRetries)
 				sleepAntigravityBackoff(attempt)
 				continue
 			}
+			// 所有重试都失败，标记限流状态
+			if resp.StatusCode == 429 {
+				s.handleUpstreamError(ctx, account, resp.StatusCode, resp.Header, respBody)
+			}
 			// 最后一次尝试也失败
 			resp = &http.Response{
 				StatusCode: resp.StatusCode,