Refund a checkout session

Refund all or part of a paid session. Send an `Idempotency-Key` header: retrying with the same key returns the first refund instead of refunding twice. Answers 201 with `{data: {id, session_id, amount, currency, status, provider_ref}}` where status is succeeded, pending (the provider is still processing) or failed (the provider refused; nothing was refunded). Errors: 404 not_found, 409 not_refundable (session not paid) / over_refund / idempotency_key_reused, 422 invalid_amount, 501 refund_unsupported (a live payment whose provider cannot refund through its API). Sends a `payment.refunded` webhook.