refactor: extract failover error handling into shared HandleFailoverError

- Extract duplicated failover error handling from gateway_handler.go (Gemini-compat & Claude paths) and gemini_v1beta_handler.go into shared failover_loop.go
- Introduce TempUnscheduler interface for testability (GatewayService implicitly satisfies it)
- Add comprehensive unit tests for HandleFailoverError (32 test cases covering all paths)
- Fix golangci-lint issues: errcheck in test type assertion, staticcheck QF1003 if/else→switch

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]")
 	}
 }