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/plugin/config.ts

@@ -19,6 +19,26 @@ export interface TestProviderConfig {
   simulateFailures?: boolean
 }
 
+export interface AdvancedTestProviderConfig {
+  enabled: boolean
+  scenarios?: Array<{
+    id: string
+    name: string
+    description: string
+    outcome: 'paid' | 'failed' | 'cancelled' | 'expired' | 'pending'
+    delay?: number
+    method?: 'ideal' | 'creditcard' | 'paypal' | 'applepay' | 'banktransfer'
+  }>
+  customUiRoute?: string
+  testModeIndicators?: {
+    showWarningBanners?: boolean
+    showTestBadges?: boolean
+    consoleWarnings?: boolean
+  }
+  defaultDelay?: number
+  baseUrl?: string
+}
+
 // Customer info extractor callback type
 export interface CustomerInfoExtractor {
   (customer: any): {