Returns the current state of the transaction (order) identified by the
supplied Merchant Unique Reference, scoped to the merchant resolved from
the M2M bearer token.

Resolution flow for the transaction details:

1. The merchant's clientId is extracted from the JWT and used to look
   up the configured store properties (storeId, authorisedProducts).
2. Orders matching the supplied transactionId (Merchant Unique
   Reference) are fetched for that store.
3. The first matching application across the merchant's authorised
   products is selected.
4. The full order is loaded from the order service and enriched with:
   • refunds – any refunds recorded against the order;
   • appLinks – AppLinks issued for the order (assisted journeys);
   • the latest status, subStatus and displayStatus.

Special status mappings applied by the service:

• Order status PART_REFUNDED → CheckoutStatus.part_refunded.
• Order status FULL_REFUNDED → CheckoutStatus.fully_refunded.
• If any application has sub-status
  COMPLETE.REFUND.REFUND_UNSUCCESSFUL, the checkout status is
  overridden with that application's status.

If no order can be resolved for the supplied reference and authenticated
merchant, 404 Not Found is returned.