fix(gemini): 完善gemini号池模式时retryable失效问题

此前 Gemini 三条转发路径对池模式账号命中 ErrorPolicySkipped 时直接把
上游错误体透传给客户端（强制标 500），既不同账号重试也不换号，
pool_mode_retry_count 配置完全不生效；而 Anthropic/OpenAI 等路径
会构造 UpstreamFailoverError 交给 handler 层按池模式配置重试后换号。

- 新增 poolModeSkippedFailoverError：池模式 + 可 failover 状态码时
  返回 UpstreamFailoverError，RetryableOnSameAccount 按
  pool_mode_retry_status_codes（默认 401/403/429）判定
- 原生 v1beta（ForwardNative）与 Claude messages 兼容路径的
  Skipped 分支接入；非池模式的自定义错误码透传行为不变
- chat completions 兼容路径的 failover 错误补上
  RetryableOnSameAccount 池模式判定

backend/internal/service/gemini_chat_completions_compat_service.go

@@ -231,7 +231,11 @@ func (s *GeminiMessagesCompatService) forwardClaudeBodyAsChatCompletions(
 				Kind:               "failover",
 				Message:            upstreamMsg,
 			})
-			return nil, &UpstreamFailoverError{StatusCode: resp.StatusCode, ResponseBody: evBody}
+			return nil, &UpstreamFailoverError{
+				StatusCode:             resp.StatusCode,
+				ResponseBody:           evBody,
+				RetryableOnSameAccount: account.IsPoolMode() && account.IsPoolModeRetryableStatus(resp.StatusCode),
+			}
 		}
 
 		return nil, s.writeGeminiChatCompletionsMappedError(c, account, resp.StatusCode, requestID, evBody)

backend/internal/service/gemini_error_policy_test.go

@@ -352,6 +352,68 @@ func TestGeminiErrorPolicyIntegration(t *testing.T) {
 	}
 }
 
+// ---------------------------------------------------------------------------
+// TestPoolModeSkippedFailoverError — pool-mode accounts hitting
+// ErrorPolicySkipped must failover (align with other platform forwards)
+// instead of passing the upstream error through to the client.
+// ---------------------------------------------------------------------------
+
+func TestPoolModeSkippedFailoverError(t *testing.T) {
+	gin.SetMode(gin.TestMode)
+	svc := &GeminiMessagesCompatService{}
+
+	poolAccount := func(extra map[string]any) *Account {
+		creds := map[string]any{"pool_mode": true}
+		for k, v := range extra {
+			creds[k] = v
+		}
+		return &Account{ID: 300, Type: AccountTypeAPIKey, Platform: PlatformGemini, Credentials: creds}
+	}
+
+	tests := []struct {
+		name              string
+		account           *Account
+		statusCode        int
+		expectFailover    bool
+		expectSameAccount bool
+	}{
+		{"pool_500_failover_no_same_account_retry", poolAccount(nil), 500, true, false},
+		{"pool_429_failover_with_same_account_retry", poolAccount(nil), 429, true, true},
+		{"pool_custom_retry_codes_500", poolAccount(map[string]any{
+			"pool_mode_retry_status_codes": []any{float64(500)},
+		}), 500, true, true},
+		{"pool_400_not_failover_worthy", poolAccount(nil), 400, false, false},
+		{"non_pool_account_keeps_passthrough", &Account{
+			ID: 301, Type: AccountTypeAPIKey, Platform: PlatformGemini,
+			Credentials: map[string]any{
+				"custom_error_codes_enabled": true,
+				"custom_error_codes":         []any{float64(429)},
+			},
+		}, 500, false, false},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			writer := httptest.NewRecorder()
+			c, _ := gin.CreateTestContext(writer)
+			c.Request = httptest.NewRequest(http.MethodPost, "/v1/messages", nil)
+
+			body := []byte(`{"error":{"code":"bad_response_status_code","message":"openai_error"}}`)
+			failoverErr := svc.poolModeSkippedFailoverError(c, tt.account, tt.statusCode, body, "req-1")
+
+			if !tt.expectFailover {
+				require.Nil(t, failoverErr)
+				return
+			}
+			require.NotNil(t, failoverErr)
+			require.Equal(t, tt.statusCode, failoverErr.StatusCode)
+			require.Equal(t, body, failoverErr.ResponseBody)
+			require.Equal(t, tt.expectSameAccount, failoverErr.RetryableOnSameAccount)
+			require.True(t, failoverErr.ShouldRetryNextAccount())
+		})
+	}
+}
+
 // ---------------------------------------------------------------------------
 // TestGeminiErrorPolicy_NilRateLimitService — verifies nil safety
 // ---------------------------------------------------------------------------

backend/internal/service/gemini_messages_compat_service.go

@@ -945,6 +945,9 @@ func (s *GeminiMessagesCompatService) Forward(ctx context.Context, c *gin.Contex
 				if upstreamReqID == "" {
 					upstreamReqID = resp.Header.Get("x-goog-request-id")
 				}
+				if failoverErr := s.poolModeSkippedFailoverError(c, account, resp.StatusCode, respBody, upstreamReqID); failoverErr != nil {
+					return nil, failoverErr
+				}
 				return nil, s.writeGeminiMappedError(c, account, http.StatusInternalServerError, upstreamReqID, respBody)
 			case ErrorPolicyMatched, ErrorPolicyTempUnscheduled:
 				if policy == ErrorPolicyMatched {
@@ -1452,6 +1455,9 @@ func (s *GeminiMessagesCompatService) ForwardNative(ctx context.Context, c *gin.
 			policy := s.rateLimitService.CheckErrorPolicy(ctx, account, resp.StatusCode, respBody, mappedModel)
 			switch policy {
 			case ErrorPolicySkipped:
+				if failoverErr := s.poolModeSkippedFailoverError(c, account, resp.StatusCode, respBody, requestID); failoverErr != nil {
+					return nil, failoverErr
+				}
 				respBody = unwrapIfNeeded(isOAuth, respBody)
 				contentType := resp.Header.Get("Content-Type")
 				if contentType == "" {
@@ -1683,6 +1689,39 @@ func (s *GeminiMessagesCompatService) shouldFailoverGeminiUpstreamError(statusCo
 	}
 }
 
+// poolModeSkippedFailoverError 池模式账号命中 ErrorPolicySkipped 时构造 failover 错误：
+// 可 failover 的状态码返回 UpstreamFailoverError，交给 handler 层按 pool_mode_retry_count
+// 同账号重试后换号；返回 nil 表示不适用（非池模式或状态码不可 failover），由调用方透传。
+func (s *GeminiMessagesCompatService) poolModeSkippedFailoverError(c *gin.Context, account *Account, statusCode int, respBody []byte, upstreamRequestID string) *UpstreamFailoverError {
+	if !account.IsPoolMode() || !s.shouldFailoverGeminiUpstreamError(statusCode) {
+		return nil
+	}
+	upstreamMsg := sanitizeUpstreamErrorMessage(strings.TrimSpace(extractUpstreamErrorMessage(respBody)))
+	upstreamDetail := ""
+	if s.cfg != nil && s.cfg.Gateway.LogUpstreamErrorBody {
+		maxBytes := s.cfg.Gateway.LogUpstreamErrorBodyMaxBytes
+		if maxBytes <= 0 {
+			maxBytes = 2048
+		}
+		upstreamDetail = truncateString(string(respBody), maxBytes)
+	}
+	appendOpsUpstreamError(c, OpsUpstreamErrorEvent{
+		Platform:           account.Platform,
+		AccountID:          account.ID,
+		AccountName:        account.Name,
+		UpstreamStatusCode: statusCode,
+		UpstreamRequestID:  upstreamRequestID,
+		Kind:               "failover",
+		Message:            upstreamMsg,
+		Detail:             upstreamDetail,
+	})
+	return &UpstreamFailoverError{
+		StatusCode:             statusCode,
+		ResponseBody:           respBody,
+		RetryableOnSameAccount: account.IsPoolModeRetryableStatus(statusCode),
+	}
+}
+
 func sleepGeminiBackoff(attempt int) {
 	delay := geminiRetryBaseDelay * time.Duration(1<<uint(attempt-1))
 	if delay > geminiRetryMaxDelay {