Cancels a verification that is still pending so it no longer blocks a new request (PENDING_VERIFICATION_EXISTS) and a corrected one can be submitted immediately. Returns the cancelled record; calling it again on an already-cancelled verification returns the same record (idempotent). Returns 404 when the verification does not exist for this supplier and 409 with reason_code VERIFICATION_NOT_PENDING when it already completed. Never changes the supplier's payment method or stored bank data.