fix: 修复 CI 检查失败

- gofmt: 修复 error_passthrough_service.go 格式问题
- errcheck: 修复 error_passthrough_runtime_test.go 类型断言未检查
- staticcheck: if-else 改为 switch (gateway_service.go)
- test: 修复两个测试用例错误使用 MODEL_CAPACITY_EXHAUSTED 导致走错路径

backend/internal/service/gateway_service.go

@@ -385,9 +385,10 @@ func (s *GatewayService) TempUnscheduleRetryableError(ctx context.Context, accou
 		return
 	}
 	// 根据状态码选择封禁策略
-	if failoverErr.StatusCode == http.StatusBadRequest {
+	switch failoverErr.StatusCode {
+	case http.StatusBadRequest:
 		tempUnscheduleGoogleConfigError(ctx, s.accountRepo, accountID, "[handler]")
-	} else if failoverErr.StatusCode == http.StatusBadGateway {
+	case http.StatusBadGateway:
 		tempUnscheduleEmptyResponse(ctx, s.accountRepo, accountID, "[handler]")
 	}
 }