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"})
 

backend/plugins/domain/message_gateway/plugin_test.go

@@ -43,3 +43,19 @@ func TestMessageGatewayPluginUnit(t *testing.T) {
 	require.True(t, ok)
 	assert.Equal(t, 5, setting.Default)
 }
+
+// TestEveryScheduleHasTaskHandler 回归：RegisterCron 仅登记调度；若同名任务从未
+// Register，则每次触发都投递到无人处理的任务类型，清理逻辑静默失效。
+func TestEveryScheduleHasTaskHandler(t *testing.T) {
+	ctx := core.NewContext(context.Background())
+	require.NoError(t, message_gateway.New().Apply(ctx))
+
+	schedules := ctx.Schedules().Schedules()
+	require.NotEmpty(t, schedules)
+
+	for _, sched := range schedules {
+		_, ok := ctx.Tasks().Get(sched.TaskType)
+		assert.Truef(t, ok, "schedule %q dispatches to task %q, which is never registered",
+			sched.Spec, sched.TaskType)
+	}
+}