refactor(antigravity): unify TestConnection with dispatch retry loop

TestConnection now reuses antigravityRetryLoop instead of a standalone
HTTP loop, gaining credits overages, smart retry, and 429/503 backoff
for free. AccountSwitchError is caught and surfaced as a friendly
message. Also populates RateLimitedModel in TempUnscheduled switch error.

Test fixes:
- Use RATE_LIMIT_EXCEEDED in 503 short-delay test to avoid 60x1s timeout
- Clamp waitDuration=0 instead of 999s to avoid 15s max-wait timeout
- Enhance mockSmartRetryUpstream with repeatLast and body caching

backend/internal/service/antigravity_gateway_service.go

@@ -930,7 +930,7 @@ func (s *AntigravityGatewayService) applyErrorPolicy(p antigravityRetryLoopParam
 	case ErrorPolicyTempUnscheduled:
 		slog.Info("temp_unschedulable_matched",
 			"prefix", p.prefix, "status_code", statusCode, "account_id", p.account.ID)
-		return true, statusCode, &AntigravityAccountSwitchError{OriginalAccountID: p.account.ID, IsStickySession: p.isStickySession}
+		return true, statusCode, &AntigravityAccountSwitchError{OriginalAccountID: p.account.ID, RateLimitedModel: p.requestedModel, IsStickySession: p.isStickySession}
 	}
 	return false, statusCode, nil
 }
@@ -1001,8 +1001,9 @@ type TestConnectionResult struct {
 	MappedModel string // 实际使用的模型
 }
 
-// TestConnection 测试 Antigravity 账号连接（非流式，无重试、无计费）
-// 支持 Claude 和 Gemini 两种协议，根据 modelID 前缀自动选择
+// TestConnection 测试 Antigravity 账号连接。
+// 复用 antigravityRetryLoop 的完整重试 / credits overages / 智能重试逻辑，
+// 与真实调度行为一致。差异：不做账号切换（测试指定账号）、不记录 ops 错误。
 func (s *AntigravityGatewayService) TestConnection(ctx context.Context, account *Account, modelID string) (*TestConnectionResult, error) {
 
 	// 获取 token
@@ -1026,10 +1027,8 @@ func (s *AntigravityGatewayService) TestConnection(ctx context.Context, account
 	// 构建请求体
 	var requestBody []byte
 	if strings.HasPrefix(modelID, "gemini-") {
-		// Gemini 模型：直接使用 Gemini 格式
 		requestBody, err = s.buildGeminiTestRequest(projectID, mappedModel)
 	} else {
-		// Claude 模型：使用协议转换
 		requestBody, err = s.buildClaudeTestRequest(projectID, mappedModel)
 	}
 	if err != nil {
@@ -1042,64 +1041,63 @@ func (s *AntigravityGatewayService) TestConnection(ctx context.Context, account
 		proxyURL = account.Proxy.URL()
 	}
 
-	baseURL := resolveAntigravityForwardBaseURL()
-	if baseURL == "" {
-		return nil, errors.New("no antigravity forward base url configured")
-	}
-	availableURLs := []string{baseURL}
-
-	var lastErr error
-	for urlIdx, baseURL := range availableURLs {
-		// 构建 HTTP 请求（总是使用流式 endpoint，与官方客户端一致）
-		req, err := antigravity.NewAPIRequestWithURL(ctx, baseURL, "streamGenerateContent", accessToken, requestBody)
-		if err != nil {
-			lastErr = err
-			continue
-		}
-
-		// 调试日志：Test 请求信息
-		logger.LegacyPrintf("service.antigravity_gateway", "[antigravity-Test] account=%s request_size=%d url=%s", account.Name, len(requestBody), req.URL.String())
-
-		// 发送请求
-		resp, err := s.httpUpstream.Do(req, proxyURL, account.ID, account.Concurrency)
-		if err != nil {
-			lastErr = fmt.Errorf("请求失败: %w", err)
-			if shouldAntigravityFallbackToNextURL(err, 0) && urlIdx < len(availableURLs)-1 {
-				logger.LegacyPrintf("service.antigravity_gateway", "[antigravity-Test] URL fallback: %s -> %s", baseURL, availableURLs[urlIdx+1])
-				continue
-			}
-			return nil, lastErr
-		}
-
-		// 读取响应
-		respBody, err := io.ReadAll(io.LimitReader(resp.Body, 2<<20))
-		_ = resp.Body.Close() // 立即关闭，避免循环内 defer 导致的资源泄漏
-		if err != nil {
-			return nil, fmt.Errorf("读取响应失败: %w", err)
-		}
-
-		// 检查是否需要 URL 降级
-		if shouldAntigravityFallbackToNextURL(nil, resp.StatusCode) && urlIdx < len(availableURLs)-1 {
-			logger.LegacyPrintf("service.antigravity_gateway", "[antigravity-Test] URL fallback (HTTP %d): %s -> %s", resp.StatusCode, baseURL, availableURLs[urlIdx+1])
-			continue
-		}
-
-		if resp.StatusCode >= 400 {
-			return nil, fmt.Errorf("API 返回 %d: %s", resp.StatusCode, string(respBody))
-		}
-
-		// 解析流式响应，提取文本
-		text := extractTextFromSSEResponse(respBody)
-
-		// 标记成功的 URL，下次优先使用
-		antigravity.DefaultURLAvailability.MarkSuccess(baseURL)
-		return &TestConnectionResult{
-			Text:        text,
-			MappedModel: mappedModel,
-		}, nil
+	// 复用 antigravityRetryLoop：完整的重试 / credits overages / 智能重试
+	prefix := fmt.Sprintf("[antigravity-Test] account=%d(%s)", account.ID, account.Name)
+	p := antigravityRetryLoopParams{
+		ctx:            ctx,
+		prefix:         prefix,
+		account:        account,
+		proxyURL:       proxyURL,
+		accessToken:    accessToken,
+		action:         "streamGenerateContent",
+		body:           requestBody,
+		c:              nil, // 无 gin.Context → 跳过 ops 追踪
+		httpUpstream:   s.httpUpstream,
+		settingService: s.settingService,
+		accountRepo:    s.accountRepo,
+		requestedModel: modelID,
+		handleError:    testConnectionHandleError,
 	}
 
-	return nil, lastErr
+	result, err := s.antigravityRetryLoop(p)
+	if err != nil {
+		// AccountSwitchError → 测试时不切换账号，返回友好提示
+		var switchErr *AntigravityAccountSwitchError
+		if errors.As(err, &switchErr) {
+			return nil, fmt.Errorf("该账号模型 %s 当前限流中，请稍后重试", switchErr.RateLimitedModel)
+		}
+		return nil, err
+	}
+
+	if result == nil || result.resp == nil {
+		return nil, errors.New("upstream returned empty response")
+	}
+	defer func() { _ = result.resp.Body.Close() }()
+
+	respBody, err := io.ReadAll(io.LimitReader(result.resp.Body, 2<<20))
+	if err != nil {
+		return nil, fmt.Errorf("读取响应失败: %w", err)
+	}
+
+	if result.resp.StatusCode >= 400 {
+		return nil, fmt.Errorf("API 返回 %d: %s", result.resp.StatusCode, string(respBody))
+	}
+
+	text := extractTextFromSSEResponse(respBody)
+	return &TestConnectionResult{Text: text, MappedModel: mappedModel}, nil
+}
+
+// testConnectionHandleError 是 TestConnection 使用的轻量 handleError 回调。
+// 仅记录日志，不做 ops 错误追踪或粘性会话清除。
+func testConnectionHandleError(
+	_ context.Context, prefix string, account *Account,
+	statusCode int, _ http.Header, body []byte,
+	requestedModel string, _ int64, _ string, _ bool,
+) *handleModelRateLimitResult {
+	logger.LegacyPrintf("service.antigravity_gateway",
+		"%s test_handle_error status=%d model=%s account=%d body=%s",
+		prefix, statusCode, requestedModel, account.ID, truncateForLog(body, 200))
+	return nil
 }
 
 // buildGeminiTestRequest 构建 Gemini 格式测试请求