feat: implement advanced test provider with interactive UI and multiple scenarios

- Add comprehensive test provider with configurable payment outcomes (paid, failed, cancelled, expired, pending)
- Support multiple payment methods (iDEAL, Credit Card, PayPal, Apple Pay, Bank Transfer)
- Interactive test payment UI with responsive design and real-time processing simulation
- Test mode indicators including warning banners, badges, and console warnings
- React components for admin UI integration (TestModeWarningBanner, TestModeBadge, TestPaymentControls)
- API endpoints for test payment processing and status polling
- Configurable scenarios with custom delays and outcomes
- Production safety mechanisms and clear test mode indicators
- Complete documentation and usage examples

Implements GitHub issue #20 for advanced test provider functionality.

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

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

src/providers/index.ts

@@ -1,4 +1,10 @@
 export * from './mollie.js'
 export * from './stripe.js'
+export * from './test.js'
 export * from './types.js'
 export * from './currency.js'
+
+// Re-export provider configurations and types
+export type { StripeProviderConfig } from './stripe.js'
+export type { MollieProviderConfig } from './mollie.js'
+export type { TestProviderConfig, PaymentOutcome, PaymentMethod, PaymentScenario } from './test.js'