Optional free-text reason. For BACS_DD payment methods, flows to the L&Z audit
trail and the mandate CancellationRequested event. Must be ≤ 300 characters
and non-blank when set (Bacs constraint — L&Z SuspendCustomer.Reason cannot
be solely whitespace and silently truncates beyond 300 characters). Rails
other than BACS_DD ignore this field today.