Behavior: Marks the payment transaction identified by the specified ID as complete. This API call is optional and is used to help move the UI along. The actual successful status comes directly from the merchant processor in the form of a callback. 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 completion is recorded. The definitive completion status is determined by the merchant processor callback.

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.