fix 第一次 400，第二次触发切账号信号

backend/internal/service/antigravity_gateway_service.go

@@ -2241,6 +2241,20 @@ func (s *AntigravityGatewayService) ForwardGemini(ctx context.Context, c *gin.Co
 						contentType = resp.Header.Get("Content-Type")
 					}
 				} else {
+					if switchErr, ok := IsAntigravityAccountSwitchError(retryErr); ok {
+						appendOpsUpstreamError(c, OpsUpstreamErrorEvent{
+							Platform:           account.Platform,
+							AccountID:          account.ID,
+							AccountName:        account.Name,
+							UpstreamStatusCode: http.StatusServiceUnavailable,
+							Kind:               "failover",
+							Message:            sanitizeUpstreamErrorMessage(retryErr.Error()),
+						})
+						return nil, &UpstreamFailoverError{
+							StatusCode:        http.StatusServiceUnavailable,
+							ForceCacheBilling: switchErr.IsStickySession,
+						}
+					}
 					appendOpsUpstreamError(c, OpsUpstreamErrorEvent{
 						Platform:           account.Platform,
 						AccountID:          account.ID,

backend/internal/service/antigravity_gateway_service_test.go

@@ -139,6 +139,7 @@ type queuedHTTPUpstreamStub struct {
 	errors        []error
 	requestBodies [][]byte
 	callCount     int
+	onCall        func(*http.Request, *queuedHTTPUpstreamStub)
 }
 
 func (s *queuedHTTPUpstreamStub) Do(req *http.Request, _ string, _ int64, _ int) (*http.Response, error) {
@@ -152,6 +153,9 @@ func (s *queuedHTTPUpstreamStub) Do(req *http.Request, _ string, _ int64, _ int)
 
 	idx := s.callCount
 	s.callCount++
+	if s.onCall != nil {
+		s.onCall(req, s)
+	}
 
 	var resp *http.Response
 	if idx < len(s.responses) {
@@ -679,6 +683,91 @@ func TestAntigravityGatewayService_ForwardGemini_RetriesCorruptedThoughtSignatur
 	require.Equal(t, "signature_error", events[0].Kind)
 }
 
+func TestAntigravityGatewayService_ForwardGemini_SignatureRetryPropagatesFailover(t *testing.T) {
+	gin.SetMode(gin.TestMode)
+	writer := httptest.NewRecorder()
+	c, _ := gin.CreateTestContext(writer)
+
+	body, err := json.Marshal(map[string]any{
+		"contents": []map[string]any{
+			{"role": "user", "parts": []map[string]any{{"text": "hello"}}},
+			{"role": "model", "parts": []map[string]any{{"text": "thinking", "thought": true, "thoughtSignature": "sig_bad_1"}}},
+		},
+	})
+	require.NoError(t, err)
+
+	req := httptest.NewRequest(http.MethodPost, "/antigravity/v1beta/models/gemini-3.1-pro-preview:streamGenerateContent", bytes.NewReader(body))
+	c.Request = req
+
+	firstRespBody := []byte(`{"response":{"error":{"code":400,"message":"Corrupted thought signature.","status":"INVALID_ARGUMENT"}}}`)
+
+	const originalModel = "gemini-3.1-pro-preview"
+	const mappedModel = "gemini-3.1-pro-high"
+	account := &Account{
+		ID:          8,
+		Name:        "acc-gemini-signature-failover",
+		Platform:    PlatformAntigravity,
+		Type:        AccountTypeOAuth,
+		Status:      StatusActive,
+		Concurrency: 1,
+		Credentials: map[string]any{
+			"access_token": "token",
+			"model_mapping": map[string]any{
+				originalModel: mappedModel,
+			},
+		},
+	}
+
+	upstream := &queuedHTTPUpstreamStub{
+		responses: []*http.Response{
+			{
+				StatusCode: http.StatusBadRequest,
+				Header: http.Header{
+					"Content-Type": []string{"application/json"},
+					"X-Request-Id": []string{"req-sig-failover-1"},
+				},
+				Body: io.NopCloser(bytes.NewReader(firstRespBody)),
+			},
+		},
+		onCall: func(_ *http.Request, stub *queuedHTTPUpstreamStub) {
+			if stub.callCount != 1 {
+				return
+			}
+			futureResetAt := time.Now().Add(30 * time.Second).Format(time.RFC3339)
+			account.Extra = map[string]any{
+				modelRateLimitsKey: map[string]any{
+					mappedModel: map[string]any{
+						"rate_limit_reset_at": futureResetAt,
+					},
+				},
+			}
+		},
+	}
+
+	svc := &AntigravityGatewayService{
+		settingService: NewSettingService(&antigravitySettingRepoStub{}, &config.Config{Gateway: config.GatewayConfig{MaxLineSize: defaultMaxLineSize}}),
+		tokenProvider:  &AntigravityTokenProvider{},
+		httpUpstream:   upstream,
+	}
+
+	result, err := svc.ForwardGemini(context.Background(), c, account, originalModel, "streamGenerateContent", true, body, true)
+	require.Nil(t, result)
+
+	var failoverErr *UpstreamFailoverError
+	require.ErrorAs(t, err, &failoverErr, "signature retry should propagate failover instead of falling back to the original 400")
+	require.Equal(t, http.StatusServiceUnavailable, failoverErr.StatusCode)
+	require.True(t, failoverErr.ForceCacheBilling)
+	require.Len(t, upstream.requestBodies, 1, "retry should stop at preflight failover and not issue a second upstream request")
+
+	raw, ok := c.Get(OpsUpstreamErrorsKey)
+	require.True(t, ok)
+	events, ok := raw.([]*OpsUpstreamErrorEvent)
+	require.True(t, ok)
+	require.Len(t, events, 2)
+	require.Equal(t, "signature_error", events[0].Kind)
+	require.Equal(t, "failover", events[1].Kind)
+}
+
 // TestStreamUpstreamResponse_UsageAndFirstToken
 // 验证：usage 字段可被累积/覆盖更新，并且能记录首 token 时间
 func TestStreamUpstreamResponse_UsageAndFirstToken(t *testing.T) {