fix: resolve CI lint errors and test compilation failures for rate limit feature

- Fix errcheck: properly handle rows.Close() error via named return + defer closure
- Fix gofmt: auto-format billing_cache.go, api_key_service.go, billing_cache_service.go
- Add missing rate limit interface methods to 4 test stubs (GetRateLimitData, IncrementRateLimitUsage, ResetRateLimitWindows)
- Fix NewBillingCacheService calls missing the new apiKeyRepo parameter

backend/internal/handler/gateway_handler_warmup_intercept_unit_test.go

@@ -159,7 +159,7 @@ func newTestGatewayHandler(t *testing.T, group *service.Group, accounts []*servi
 
 	// RunModeSimple：跳过计费检查，避免引入 repo/cache 依赖。
 	cfg := &config.Config{RunMode: config.RunModeSimple}
-	billingCacheSvc := service.NewBillingCacheService(nil, nil, nil, cfg)
+	billingCacheSvc := service.NewBillingCacheService(nil, nil, nil, nil, cfg)
 
 	concurrencySvc := service.NewConcurrencyService(&fakeConcurrencyCache{})
 	concurrencyHelper := NewConcurrencyHelper(concurrencySvc, SSEPingFormatClaude, 0)

backend/internal/handler/sora_client_handler_test.go

@@ -1032,6 +1032,15 @@ func (r *stubAPIKeyRepoForHandler) IncrementQuotaUsed(_ context.Context, _ int64
 func (r *stubAPIKeyRepoForHandler) UpdateLastUsed(context.Context, int64, time.Time) error {
 	return nil
 }
+func (r *stubAPIKeyRepoForHandler) IncrementRateLimitUsage(context.Context, int64, float64) error {
+	return nil
+}
+func (r *stubAPIKeyRepoForHandler) ResetRateLimitWindows(context.Context, int64) error {
+	return nil
+}
+func (r *stubAPIKeyRepoForHandler) GetRateLimitData(context.Context, int64) (*service.APIKeyRateLimitData, error) {
+	return nil, nil
+}
 
 // newTestAPIKeyService 创建测试用的 APIKeyService
 func newTestAPIKeyService(repo *stubAPIKeyRepoForHandler) *service.APIKeyService {

backend/internal/handler/sora_gateway_handler_test.go

@@ -411,7 +411,7 @@ func TestSoraGatewayHandler_ChatCompletions(t *testing.T) {
 	deferredService := service.NewDeferredService(accountRepo, nil, 0)
 	billingService := service.NewBillingService(cfg, nil)
 	concurrencyService := service.NewConcurrencyService(testutil.StubConcurrencyCache{})
-	billingCacheService := service.NewBillingCacheService(nil, nil, nil, cfg)
+	billingCacheService := service.NewBillingCacheService(nil, nil, nil, nil, cfg)
 	t.Cleanup(func() {
 		billingCacheService.Stop()
 	})