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/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()
 	})