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/index.ts

@@ -1,8 +1,17 @@
 
 export { billingPlugin } from './plugin/index.js'
 export { mollieProvider, stripeProvider } from './providers/index.js'
-export type { BillingPluginConfig, CustomerInfoExtractor } from './plugin/config.js'
+export type { BillingPluginConfig, CustomerInfoExtractor, AdvancedTestProviderConfig } from './plugin/config.js'
 export type { Invoice, Payment, Refund } from './plugin/types/index.js'
 export type { PaymentProvider, ProviderData } from './providers/types.js'
-export type { MollieProviderConfig } from './providers/mollie.js'
-export type { StripeProviderConfig } from './providers/stripe.js'
+
+// Export all providers
+export { testProvider } from './providers/test.js'
+export type {
+  StripeProviderConfig,
+  MollieProviderConfig,
+  TestProviderConfig,
+  PaymentOutcome,
+  PaymentMethod,
+  PaymentScenario
+} from './providers/index.js'