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/router/ns_watcher.go

@@ -25,7 +25,9 @@ func StartNSWatcher(
 	ts *HTTPTriggerSet,
 	mgr manager.Interface,
 ) {
-	_, err := utils.RunManagedNamespaceWatcher(ctx, logger, kubeClient, mgr, utils.NewDefaultManagedNamespaceWatcherConfig("router.NSWatcher", NewNamespaceSubscriber(ts, mgr)))
+	config := utils.NewDefaultManagedNamespaceWatcherConfig("router.NSWatcher", NewNamespaceSubscriber(ts, mgr))
+	config.RemovalStrategy = utils.NamespaceRemovalStrategyDispatchRemove
+	_, err := utils.RunManagedNamespaceWatcher(ctx, logger, kubeClient, mgr, config)
 	if err != nil {
 		logger.Error("router.NSWatcher: BootstrapAndDispatch failed", zap.Error(err))
 	}