post
https://apis.kentro.live/store/v1/checkout.cancel
Cancels the attempt (attemptId) and releases its reservations, only after the provider confirms that no payment can complete; an attempt whose payment is deciding is refused with PAYMENT_PENDING. The cart is writable again.
Who may call it: The identity that started the attempt: the signed-in buyer, or the guest (send the same X-Storefront-Guest and guestAttempt: true). A signed-in buyer recovering an attempt started as a guest sends both tokens.
