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)