message_gateway scheduled message_gateway:cleanup_pairing_codes every 10
minutes but never registered a task under that pattern, so every dispatch
went to a task type with no handler and expired pairing rows accumulated
forever, even though repository.DeleteExpiredPairingCodes already existed.
Add a test that fails for any schedule whose task pattern is unregistered:
it reports the exact orphan rather than relying on a schedule-exists assert.
ryan