Use this only when the customer has explicitly asked to cancel their transfer request and you have the booking reference returned by create_transfer_request and the email used for it. Cancels a request that is still pending or assigned, tells the operator (WhatsApp/email) and emails the customer a cancellation notice. Calling it again for an already-cancelled request is harmless and returns cancelled=true without notifying anyone again. Destructive and sends real messages: never call it to explore, to "test", or because the customer is merely unsure or wants to change details. Do not use for a request the operator has already confirmed (it is refused: the customer must contact the operator), for changing time/passengers/vehicle (contact the operator), or for refunds. Use get_request_status to check the current state first if unsure.