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_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 {