fix(namespace): executor/router/buildermgr RemoveNamespace + per-NS informer lifecycle

- Add RemoveNamespace(ctx, ns) to executortype.ExecutorType interface
- Implement RemoveNamespace in poolmgr, newdeploy, container executor types
- Add per-namespace context cancellation (nsCancels map) in all three types so
  informer factories are stopped when namespace is removed (fixes goroutine leak)
- Add PoolPodController.RemoveNamespace to clear envLister/podLister maps
- Add deregisterNamespace() in executor multitenant subscriber
- Switch executor/router/buildermgr watcher strategy from TrackOnly to DispatchRemove
  so RemoveFunc is called when fission.io/managed label is removed
- Add RemoveFunc to executor/router/buildermgr namespace subscribers
- Add RemoveNamespace to environmentWatcher and packageWatcher with per-NS cancel
- Add RemoveNamespace to HTTPTriggerSet: cancels informers, removes from maps, calls syncTriggers
- Fix ns_watcher_test.go fakeExecutorType to implement new RemoveNamespace method

Fixes:
- Executor dedup gap: re-added namespace was silently skipped (envLister/deplLister still present)
- Goroutine/FD leak: old informer factories ran forever after namespace removal
- Router stale routes: HTTPTriggers for removed namespace stayed in routing table

pkg/buildermgr/envwatcher.go

@@ -76,6 +76,8 @@ type (
 		podSpecPatch           *apiv1.PodSpec
 		envWatchInformer       map[string]k8sCache.SharedIndexInformer
 		enableOwnerReferences  bool
+		// nsCancels holds per-namespace context cancel functions.
+		nsCancels map[string]context.CancelFunc
 	}
 )
 
@@ -111,8 +113,8 @@ func makeEnvironmentWatcher(
 		podSpecPatch:           podSpecPatch,
 		envWatchInformer:       utils.GetInformersForNamespaces(fissionClient, time.Minute*30, fv1.EnvironmentResource),
 		enableOwnerReferences:  utils.IsOwnerReferencesEnabled(),
+		nsCancels:              make(map[string]context.CancelFunc),
 	}
-
 	err := envWatcher.EnvWatchEventHandlers(ctx)
 	if err != nil {
 		return nil, err
@@ -540,6 +542,21 @@ func (envw *environmentWatcher) AddNamespace(ctx context.Context, ns string, mgr
 
 	envw.envWatchInformer[ns] = envInf
 	mgr.AddInformers(ctx, map[string]k8sCache.SharedIndexInformer{ns: envInf})
-	factory.Start(ctx.Done())
+
+	// Create a per-namespace cancellable context for informer lifecycle.
+	nsCtx, nsCancel := context.WithCancel(ctx)
+	envw.nsCancels[ns] = nsCancel
+
+	factory.Start(nsCtx.Done())
 	envw.logger.Info("buildermgr.envWatcher.AddNamespace: done", zap.String("namespace", ns))
 }
+
+// RemoveNamespace deregisters a namespace from the environment watcher.
+func (envw *environmentWatcher) RemoveNamespace(ns string) {
+	if cancel, ok := envw.nsCancels[ns]; ok {
+		cancel()
+		delete(envw.nsCancels, ns)
+	}
+	delete(envw.envWatchInformer, ns)
+	envw.logger.Info("buildermgr.envWatcher.RemoveNamespace: cleaned up", zap.String("namespace", ns))
+}