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/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[]