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/invoices.ts

@@ -5,10 +5,10 @@ import {
   CollectionBeforeValidateHook,
   CollectionConfig, Field,
 } from 'payload'
-import type { BillingPluginConfig} from '../plugin/config.js';
-import { defaults } from '../plugin/config.js'
-import { extractSlug } from '../plugin/utils.js'
-import type { Invoice } from '../plugin/types/invoices.js'
+import type { BillingPluginConfig} from '../plugin/config';
+import { defaults } from '../plugin/config'
+import { extractSlug } from '../plugin/utils'
+import type { Invoice } from '../plugin/types/invoices'
 
 export function createInvoicesCollection(pluginConfig: BillingPluginConfig): CollectionConfig {
   const {customerRelationSlug, customerInfoExtractor} = pluginConfig