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/namespace_subscriber.go

@@ -11,12 +11,22 @@ type routerNamespaceAdder interface {
 	AddNamespace(ctx context.Context, ns string, mgr manager.Interface) error
 }
 
+type routerNamespaceRemover interface {
+	RemoveNamespace(ns string)
+}
+
 func NewNamespaceSubscriber(ts routerNamespaceAdder, mgr manager.Interface) utils.NamespaceSubscriber {
 	return utils.NamespaceSubscriberFuncs{
 		SubscriberName: "router",
 		AddFunc: func(ctx context.Context, record utils.NamespaceRecord) error {
 			return registerRouterNamespace(ctx, record.Name, ts, mgr)
 		},
+		RemoveFunc: func(ctx context.Context, record utils.NamespaceRecord) error {
+			if r, ok := ts.(routerNamespaceRemover); ok {
+				r.RemoveNamespace(record.Name)
+			}
+			return nil
+		},
 		ResyncFunc: func(ctx context.Context, record utils.NamespaceRecord) error {
 			return registerRouterNamespace(ctx, record.Name, ts, mgr)
 		},