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/providers/utils.ts

@@ -1,9 +1,9 @@
 import type { Payload } from 'payload'
-import type { Payment } from '../plugin/types/payments.js'
-import type { BillingPluginConfig } from '../plugin/config.js'
-import type { ProviderData } from './types.js'
-import { defaults } from '../plugin/config.js'
-import { extractSlug, toPayloadId } from '../plugin/utils.js'
+import type { Payment } from '../plugin/types/payments'
+import type { BillingPluginConfig } from '../plugin/config'
+import type { ProviderData } from './types'
+import { defaults } from '../plugin/config'
+import { extractSlug, toPayloadId } from '../plugin/utils'
 
 /**
  * Common webhook response utilities