fix: resolve plugin initialization failure in Next.js API routes

Use Symbol.for() instead of Symbol() for plugin singleton storage to ensure
plugin state persists across different module loading contexts (admin panel,
API routes, server components).

This fixes the "Billing plugin not initialized" error that occurred when
calling payload.create() from Next.js API routes, server components, or
server actions.

Changes:
- Plugin singleton now uses Symbol.for('@xtr-dev/payload-billing')
- Provider singletons (stripe, mollie, test) use global symbols
- Enhanced error message with troubleshooting guidance

Fixes #1

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

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

src/providers/test.ts

@@ -6,7 +6,7 @@ import { handleWebhookError, logWebhookEvent } from './utils'
 import { isValidAmount, isValidCurrencyCode } from './currency'
 import { createContextLogger } from '../utils/logger'
 
-const TestModeWarningSymbol = Symbol('TestModeWarning')
+const TestModeWarningSymbol = Symbol.for('@xtr-dev/payload-billing/test-mode-warning')
 const hasGivenTestModeWarning = () => TestModeWarningSymbol in globalThis
 const setTestModeWarning = () => ((<any>globalThis)[TestModeWarningSymbol] = true)