fix: use built-in UI when customUiRoute is not specified

In v0.1.19, the fix for customUiRoute made it always use the default
route '/test-payment' even when customUiRoute was not specified. This
caused 404 errors because users were unaware of this default behavior.

The plugin actually provides a built-in test payment UI at
/api/payload-billing/test/payment/:id that works out of the box.

This fix ensures the correct behavior:
- When customUiRoute IS specified: Use the custom route
- When customUiRoute is NOT specified: Use the built-in UI route

This allows the testProvider to work out of the box without requiring
users to implement a custom test payment page, while still supporting
custom implementations when needed.

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

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

src/providers/test.ts

@@ -492,7 +492,10 @@ export const testProvider = (testConfig: TestProviderConfig) => {
 
       // Set provider ID and data
       payment.providerId = testPaymentId
-      const paymentUrl = `${baseUrl}${uiRoute}/${testPaymentId}`
+      // Use custom UI route if specified, otherwise use built-in UI endpoint
+      const paymentUrl = testConfig.customUiRoute
+        ? `${baseUrl}${testConfig.customUiRoute}/${testPaymentId}`
+        : `${baseUrl}/api/payload-billing/test/payment/${testPaymentId}`
       const providerData: ProviderData = {
         raw: {
           id: testPaymentId,