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/executor/multitenant/namespace_subscriber.go

@@ -24,6 +24,9 @@ func NewNamespaceSubscriber(
 			registerNamespace(ctx, logger, kubernetesClient, record.Name, executorTypes, mgr)
 			return nil
 		},
+		RemoveFunc: func(ctx context.Context, record utils.NamespaceRecord) error {
+			return deregisterNamespace(ctx, logger, record.Name, executorTypes)
+		},
 		ResyncFunc: func(ctx context.Context, record utils.NamespaceRecord) error {
 			registerNamespace(ctx, logger, kubernetesClient, record.Name, executorTypes, mgr)
 			return nil