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/plugin/config.ts

@@ -1,6 +1,6 @@
 import { CollectionConfig } from 'payload'
-import { FieldsOverride } from './utils.js'
-import { PaymentProvider } from './types/index.js'
+import { FieldsOverride } from './utils'
+import { PaymentProvider } from './types/index'
 
 export const defaults = {
   paymentsCollection: 'payments',
@@ -20,7 +20,7 @@ export interface TestProviderConfig {
 }
 
 // Re-export the actual test provider config instead of duplicating
-export type { TestProviderConfig as AdvancedTestProviderConfig } from '../providers/test.js'
+export type { TestProviderConfig as AdvancedTestProviderConfig } from '../providers/test'
 
 // Customer info extractor callback type
 export interface CustomerInfoExtractor {

src/plugin/index.ts

@@ -1,8 +1,8 @@
-import { createInvoicesCollection, createPaymentsCollection, createRefundsCollection } from '../collections/index.js'
-import type { BillingPluginConfig } from './config.js'
+import { createInvoicesCollection, createPaymentsCollection, createRefundsCollection } from '../collections/index'
+import type { BillingPluginConfig } from './config'
 import type { Config, Payload } from 'payload'
-import { createSingleton } from './singleton.js'
-import type { PaymentProvider } from '../providers/index.js'
+import { createSingleton } from './singleton'
+import type { PaymentProvider } from '../providers/index'
 
 const singleton = createSingleton(Symbol('billingPlugin'))
 

src/plugin/types/invoices.ts

@@ -1,5 +1,5 @@
-import { Payment } from './payments.js'
-import { Id } from './id.js'
+import { Payment } from './payments'
+import { Id } from './id'
 
 export interface Invoice<TCustomer = unknown> {
   id: Id;

src/plugin/types/payments.ts

@@ -1,6 +1,6 @@
-import { Refund } from './refunds.js'
-import { Invoice } from './invoices.js'
-import { Id } from './id.js'
+import { Refund } from './refunds'
+import { Invoice } from './invoices'
+import { Id } from './id'
 
 export interface Payment {
   id: Id;

src/plugin/types/refunds.ts

@@ -1,4 +1,4 @@
-import { Payment } from './payments.js'
+import { Payment } from './payments'
 
 export interface Refund {
   id: number;

src/plugin/utils.ts

@@ -1,5 +1,5 @@
 import type { CollectionConfig, CollectionSlug, Field } from 'payload'
-import type { Id } from './types/index.js'
+import type { Id } from './types/index'
 
 export type FieldsOverride = (args: { defaultFields: Field[] }) => Field[]