Retrieves pre-cancellation preview data for a customer's order.

Use this endpoint to populate the cancellation UI before submitting the
cancel request. The response includes:

• use_new_cancel_flow — whether to render the rich cancel popup
  (Shopify) or the legacy single-field flow (BigCommerce, Custom).
• supports_refund_on_cancel — whether the order has a paid balance
  eligible for refund on cancellation.
• cancel_reasons — the list of selectable cancellation reason options.
• refund_payments — the suggested refund amount and payment gateway
  for the original payment method.
• refundable_items — line items that would be included in the refund.

For BigCommerce and Custom integrations, use_new_cancel_flow is always
false. In that case, cancel_reasons and refundable_items are returned
as empty lists, and refund_payments is empty lists.