Behavior: Cancels the payment transaction identified by the specified ID. This should be used when the user has decided not to continue with the purchase. The Cognito contact ID is extracted from the authenticated user's claims.

Authorization: Requires an authenticated user (class-level [Authorize]). The user's Cognito contact ID claim is required.

Side Effects: The payment transaction is cancelled with the merchant processor and the transaction state is updated.

Error Scenarios: Returns 400 if the transaction ID is invalid or the Cognito contact ID claim is missing. Returns 401 if the user is not authenticated.