autoresearch iter 17: wire the pairing-code cleanup cron to a real handler

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.

backend/plugins/domain/message_gateway/plugin.go

@@ -154,6 +154,10 @@ func (p *Plugin) Apply(ctx *core.Context) error {
 		return nil
 	})
 
+	ctx.Task().Register("message_gateway:cleanup_pairing_codes", func(c context.Context, _ []byte) error {
+		return repository.DeleteExpiredPairingCodes(c)
+	}, extpoints.WithTaskRetry(defaultTaskRetry))
+
 	// 6. Register Cron Schedules
 	ctx.Schedule().RegisterCron("*/10 * * * *", "message_gateway:cleanup_pairing_codes", map[string]any{"action": "cleanup"})