fix: respect customUiRoute configuration in test provider

The test provider's customUiRoute parameter was being ignored when
generating checkout URLs. The checkout URL was always using the
hardcoded API endpoint instead of the configured custom UI route.

This fix ensures that when customUiRoute is configured, the generated
checkoutUrl will use the custom route (e.g., /test-payment/:id)
instead of the default API route.

Fixes issue where test provider checkout URLs returned 404 errors
because they pointed to /api/payload-billing/test/payment/:id instead
of the configured custom UI route.

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

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

src/providers/test.ts

@@ -492,7 +492,7 @@ export const testProvider = (testConfig: TestProviderConfig) => {
 
       // Set provider ID and data
       payment.providerId = testPaymentId
-      const paymentUrl = `${baseUrl}/api/payload-billing/test/payment/${testPaymentId}`
+      const paymentUrl = `${baseUrl}${uiRoute}/${testPaymentId}`
       const providerData: ProviderData = {
         raw: {
           id: testPaymentId,