Ping is a lightweight endpoint used to confirm that an API credential is valid.

A successful call confirms the request reached the server and the token was accepted.
The response reports the request is authenticated and which merchant it resolves to.

The Ping endpoint supports a single health check call.
A response with a merchantId confirms the API key is valid and requests can proceed.
A missing or null merchantId means the API key, client ID, or client secret should be checked.

* GET /pay-int-api/ping.
Confirms the request is authenticated and returns the merchant it resolves to.