feat: make InitPayment support both async and non-async functions

- Updated InitPayment type to return Promise<Partial<Payment>> | Partial<Payment>
- Modified initProviderPayment hook to handle both async and sync returns using Promise.resolve()
- Enables payment providers to use either async or synchronous initPayment implementations

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

src/providers/types.ts

@@ -2,7 +2,7 @@ import type { Payment } from '../plugin/types/payments'
 import type { Config, Payload } from 'payload'
 import type { BillingPluginConfig } from '../plugin/config'
 
-export type InitPayment = (payload: Payload, payment: Partial<Payment>) => Promise<Partial<Payment>>
+export type InitPayment = (payload: Payload, payment: Partial<Payment>) => Promise<Partial<Payment>> | Partial<Payment>
 
 export type PaymentProvider = {
   key: string