feat: add test provider config endpoint

Add GET /api/payload-billing/test/config endpoint to retrieve test provider configuration including scenarios, payment methods, and test mode indicators.

This allows custom UIs to dynamically sync with plugin configuration instead of hardcoding values.

- Add TestProviderConfigResponse interface
- Export new type in provider index and main index
- Endpoint returns enabled status, scenarios, methods, test mode indicators, default delay, and custom UI route

Resolves #22

Co-authored-by: Bas <bvdaakster@users.noreply.github.com>

src/index.ts

@@ -11,6 +11,7 @@ export type {
   StripeProviderConfig,
   MollieProviderConfig,
   TestProviderConfig,
+  TestProviderConfigResponse,
   PaymentOutcome,
   PaymentMethod,
   PaymentScenario