Retrieve detailed information about a specific transaction by its merchant transaction ID.

This endpoint returns complete transaction data including payment method details,
gateway response information, recovery data and status of transaction using the
merchant-provided transaction identifier.

Inline-retry resolution: platform-initiated inline retry attempts carry
platform-assigned transaction identifiers. This lookup resolves the payment submitted
under the merchant id and returns its definitive (latest) attempt, so an approval won
on a retry is found under the id the merchant submitted. In that case the returned
merchantTransactionId is the platform-assigned attempt id rather than the path value;
use the grouped view (includeAllTransactions=true) to list every attempt.

Pending intent: when the platform has accepted a payment with this
merchantTransactionId (intent recorded before gateway dispatch) but no transaction
record is visible yet, the lookup returns a PendingTransactionResponse
(state: pending) instead of 404 — hold and re-poll until it resolves to the full
transaction. A 404 means no payment with this id was ever accepted. Pending intents
are not returned on the grouped view (includeAllTransactions=true), which lists
only visible transaction records.