fix: resolve module import issues for Next.js/Turbopack compatibility

- Remove .js extensions from all TypeScript imports throughout codebase
- Update dev config to use testProvider instead of mollieProvider for testing
- Fix module resolution issues preventing development server startup
- Enable proper testing of billing plugin functionality with test provider

This resolves the "Module not found: Can't resolve" errors that were
preventing the development server from starting with Next.js/Turbopack.
All TypeScript imports now use extension-less imports as required.

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

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

src/collections/hooks.ts

@@ -1,6 +1,6 @@
-import type { Payment } from '../plugin/types/index.js'
+import type { Payment } from '../plugin/types/index'
 import type { Payload } from 'payload'
-import { useBillingPlugin } from '../plugin/index.js'
+import { useBillingPlugin } from '../plugin/index'
 
 export const initProviderPayment = (payload: Payload, payment: Partial<Payment>) => {
   const billing = useBillingPlugin(payload)