Withdraw is allowed from INGEDIEND or IN_BEHANDELING, needs no zaak, is idempotent, and is
rejected once the registration has been decided (INGESCHREVEN/AFGEWEZEN). The WithdrawRegistration
handler loads, withdraws, and persists; a repeated withdrawal is a no-op.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>