fix: count_tokens 端点不支持时降级返回空值 (404 only)

第三方 Anthropic 中转站通常不支持 /v1/messages/count_tokens 端点，
上游返回 404 时降级返回 {input_tokens: 0}，客户端 fallback 到本地估算。

- 仅匹配 404 状态码，语义明确：端点不存在
- 其他错误 (400/429/500) 保留原始处理链和 ops 遥测
- 无需解析错误消息内容，不依赖字符串匹配
- 新增 table-driven 测试覆盖 fallback 和 non-fallback 路径

backend/internal/service/gateway_anthropic_apikey_passthrough_test.go

@@ -262,6 +262,101 @@ func TestGatewayService_AnthropicAPIKeyPassthrough_ForwardCountTokensPreservesBo
 	require.Empty(t, rec.Header().Get("Set-Cookie"))
 }
 
+func TestGatewayService_AnthropicAPIKeyPassthrough_CountTokensFallbackOn404(t *testing.T) {
+	gin.SetMode(gin.TestMode)
+
+	tests := []struct {
+		name         string
+		statusCode   int
+		respBody     string
+		wantFallback bool
+	}{
+		{
+			name:         "404 endpoint not found triggers fallback",
+			statusCode:   http.StatusNotFound,
+			respBody:     `{"error":{"message":"Not found: /v1/messages/count_tokens","type":"not_found_error"}}`,
+			wantFallback: true,
+		},
+		{
+			name:         "404 generic not found triggers fallback",
+			statusCode:   http.StatusNotFound,
+			respBody:     `{"error":{"message":"resource not found","type":"not_found_error"}}`,
+			wantFallback: true,
+		},
+		{
+			name:         "400 Invalid URL does not fallback",
+			statusCode:   http.StatusBadRequest,
+			respBody:     `{"error":{"message":"Invalid URL (POST /v1/messages/count_tokens)","type":"invalid_request_error"}}`,
+			wantFallback: false,
+		},
+		{
+			name:         "400 model error does not fallback",
+			statusCode:   http.StatusBadRequest,
+			respBody:     `{"error":{"message":"model not found: claude-unknown","type":"invalid_request_error"}}`,
+			wantFallback: false,
+		},
+		{
+			name:         "500 internal error does not fallback",
+			statusCode:   http.StatusInternalServerError,
+			respBody:     `{"error":{"message":"internal error","type":"api_error"}}`,
+			wantFallback: false,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			rec := httptest.NewRecorder()
+			c, _ := gin.CreateTestContext(rec)
+			c.Request = httptest.NewRequest(http.MethodPost, "/v1/messages/count_tokens", nil)
+
+			body := []byte(`{"model":"claude-sonnet-4-5-20250929","messages":[{"role":"user","content":"hi"}]}`)
+			parsed := &ParsedRequest{Body: body, Model: "claude-sonnet-4-5-20250929"}
+
+			upstream := &anthropicHTTPUpstreamRecorder{
+				resp: &http.Response{
+					StatusCode: tt.statusCode,
+					Header:     http.Header{"Content-Type": []string{"application/json"}},
+					Body:       io.NopCloser(strings.NewReader(tt.respBody)),
+				},
+			}
+
+			svc := &GatewayService{
+				cfg: &config.Config{
+					Gateway: config.GatewayConfig{MaxLineSize: defaultMaxLineSize},
+				},
+				httpUpstream:     upstream,
+				rateLimitService: nil,
+			}
+
+			account := &Account{
+				ID:          200,
+				Name:        "proxy-acc",
+				Platform:    PlatformAnthropic,
+				Type:        AccountTypeAPIKey,
+				Concurrency: 1,
+				Credentials: map[string]any{
+					"api_key":  "sk-proxy",
+					"base_url": "https://proxy.example.com",
+				},
+				Extra:       map[string]any{"anthropic_passthrough": true},
+				Status:      StatusActive,
+				Schedulable: true,
+			}
+
+			err := svc.ForwardCountTokens(context.Background(), c, account, parsed)
+
+			if tt.wantFallback {
+				require.NoError(t, err)
+				require.Equal(t, http.StatusOK, rec.Code)
+				require.JSONEq(t, `{"input_tokens":0}`, rec.Body.String())
+			} else {
+				require.Error(t, err)
+				require.Equal(t, tt.statusCode, rec.Code)
+			}
+		})
+	}
+}
+
 func TestGatewayService_AnthropicAPIKeyPassthrough_BuildRequestRejectsInvalidBaseURL(t *testing.T) {
 	gin.SetMode(gin.TestMode)
 	rec := httptest.NewRecorder()

backend/internal/service/gateway_service.go

@@ -6199,6 +6199,16 @@ func (s *GatewayService) forwardCountTokensAnthropicAPIKeyPassthrough(ctx contex
 
 		upstreamMsg := strings.TrimSpace(extractUpstreamErrorMessage(respBody))
 		upstreamMsg = sanitizeUpstreamErrorMessage(upstreamMsg)
+
+		// 中转站不支持 count_tokens 端点时（404），降级返回空值，客户端会 fallback 到本地估算。
+		if resp.StatusCode == http.StatusNotFound {
+			logger.LegacyPrintf("service.gateway",
+				"[count_tokens] Upstream does not support count_tokens (404), returning fallback: account=%d name=%s msg=%s",
+				account.ID, account.Name, truncateString(upstreamMsg, 512))
+			c.JSON(http.StatusOK, gin.H{"input_tokens": 0})
+			return nil
+		}
+
 		upstreamDetail := ""
 		if s.cfg != nil && s.cfg.Gateway.LogUpstreamErrorBody {
 			maxBytes := s.cfg.Gateway.LogUpstreamErrorBodyMaxBytes