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/namespace_subscriber.go

@@ -11,10 +11,18 @@ type builderEnvNamespaceAdder interface {
 	AddNamespace(ctx context.Context, ns string, mgr manager.Interface)
 }
 
+type builderEnvNamespaceRemover interface {
+	RemoveNamespace(ns string)
+}
+
 type builderPkgNamespaceAdder interface {
 	AddNamespace(ctx context.Context, ns string, mgr manager.Interface)
 }
 
+type builderPkgNamespaceRemover interface {
+	RemoveNamespace(ns string)
+}
+
 func NewNamespaceSubscriber(envw builderEnvNamespaceAdder, pkgw builderPkgNamespaceAdder, mgr manager.Interface) utils.NamespaceSubscriber {
 	return utils.NamespaceSubscriberFuncs{
 		SubscriberName: "buildermgr",
@@ -22,6 +30,10 @@ func NewNamespaceSubscriber(envw builderEnvNamespaceAdder, pkgw builderPkgNamesp
 			registerBuilderNamespace(ctx, record.Name, envw, pkgw, mgr)
 			return nil
 		},
+		RemoveFunc: func(ctx context.Context, record utils.NamespaceRecord) error {
+			deregisterBuilderNamespace(record.Name, envw, pkgw)
+			return nil
+		},
 		ResyncFunc: func(ctx context.Context, record utils.NamespaceRecord) error {
 			registerBuilderNamespace(ctx, record.Name, envw, pkgw, mgr)
 			return nil
@@ -41,3 +53,16 @@ func registerBuilderNamespace(ctx context.Context, namespace string, envw builde
 		pkgw.AddNamespace(ctx, namespace, mgr)
 	}
 }
+
+func deregisterBuilderNamespace(namespace string, envw builderEnvNamespaceAdder, pkgw builderPkgNamespaceAdder) {
+	if namespace == "" {
+		return
+	}
+	utils.DefaultNSResolver().RemoveNamespace(namespace)
+	if r, ok := envw.(builderEnvNamespaceRemover); ok {
+		r.RemoveNamespace(namespace)
+	}
+	if r, ok := pkgw.(builderPkgNamespaceRemover); ok {
+		r.RemoveNamespace(namespace)
+	}
+}