feat(payments): improve VNPay, MoMo, ZaloPay services with ConfigService

Migrate payment gateway services from hardcoded config to NestJS
ConfigService injection. Improve payment handler error handling and
update gateway factory specs.

Co-Authored-By: Paperclip <noreply@paperclip.ing>

apps/api/src/modules/payments/domain/events/payment-completed.event.ts

@@ -1,5 +1,5 @@
 import { type PaymentProvider } from '@prisma/client';
-import { type DomainEvent } from '@modules/shared/domain/domain-event';
+import { type DomainEvent } from '@modules/shared';
 
 export class PaymentCompletedEvent implements DomainEvent {
   readonly eventName = 'payment.completed';