Call this after the diner completes payment and is redirected back.
This endpoint does three things in sequence:
Returns the full order object on success, or 422 with specific errors
if the cart is invalid or payment hasn’t completed yet.
If the order was already created (e.g. duplicate call), returns the existing order.
After success, the session automatically gets a new empty cart.
Channel identifier. A second header X-Session-Id is also required.
Order created successfully