docs: inconsistent receiptUrl between JS SDK and backend-java SDK

[rhctmxk]

Description

While integrating Bootpay with Next.js (frontend) and Kotlin backend (using backend-java SDK) in test mode,
I noticed that the receiptUrl returned by the SDK is inconsistent compared to the JavaScript SDK.

• JavaScript SDK (test mode):
  https://door.bootpay.co.kr/receipt/{id}

• backend-java SDK (test mode):
  https://app.bootpay.co.kr/bill/{id}

This difference (door vs app, receipt vs bill) is not documented, which may cause confusion during integration.

Expected behavior

• Both SDKs should provide a consistent receiptUrl format, or
• The documentation should clarify that the test environment may return different URLs depending on the SDK.

Steps to reproduce

1. Integrate Bootpay backend-java SDK in Kotlin backend
2. Execute payment in test mode
3. Observe the returned receiptUrl

Environment

• backend-java SDK version: 17
• Frontend: Next.js
• Backend: Kotlin
• Mode: Test PG environment

Additional context

This was observed in test environment.
If the production/live mode always returns a valid and consistent URL, it would still be helpful to document this behavior for test mode.