[ENHANCEMENT] update payments API response

[kashif-m]

This issue is around two specific fields in the payments API

1. Payments response -error_code and error_message

HyperSwitch should not propagate the intermediary dummy error codes and error messages to the API response. Currently, ErrorResponse in connector integration mandates setting an error_code and error_message. But some connectors might not have this available during transaction failures. For this reason, current HyperSwitch integration uses NO_ERROR_CODE for error_code and NO_ERROR_MESSAGE for error_message. These are constant strings which are propagated from connector integration to core layer and later sent back in the response. It doesn't make sense to have these in the API response as HyperSwitch should not 'invent' new codes and messages. This should be removed from the API response in such cases.

2. Payments request - return_url

HyperSwitch should always validate the presence of return_url during payment link creation as it is a required field in payment link flows.