fix(executor): P2 — pre-register managed NS before adopt/cleanup (closes AdoptExistingResources race)

Problem:
  executor starts → AdoptExistingResources + CleanupOldExecutorObjects run
  against utils.DefaultNSResolver().Snapshot() which returns ONLY static NS
  from FISSION_RESOURCE_NAMESPACES. Managed (labeled) namespaces are
  registered later, asynchronously, by StartNSWatcher.

  Result:
  - Pods from a previous executor in managed NS are never adopted
    (no instanceID patch) → poolmgr creates new pool pods → cold start
    for first request after executor restart.
  - Old executor objects (RS/deployments) in managed NS accumulate
    without being cleaned up (resource leak).

Fix:
  Add multitenant.PreRegisterManagedNamespaces(ctx, logger, kubernetesClient)
  called synchronously in executor.go BEFORE the adopt/cleanup goroutines.

  The function does a single Namespaces.List with label
  fission.io/managed=true and calls DefaultNSResolver().AddNamespace() for
  each result. This is idempotent with the later watcher AddFunc calls.

  Failure is non-fatal: a warning is logged and startup proceeds with
  static NS only (safe degraded mode).

  After this call DefaultNSResolver().Snapshot() includes managed NS, so:
  - AdoptExistingResources patches old pods in managed NS with new instanceID
  - CleanupOldExecutorObjects removes stale objects from managed NS
  - GetReaperNamespace() returns the full tenant NS set

Files:
  pkg/executor/multitenant/ns_watcher.go — PreRegisterManagedNamespaces()
  pkg/executor/executor.go               — call before adopt/cleanup

pkg/executor/executor.go

@@ -347,6 +347,11 @@ func StartExecutor(ctx context.Context, clientGen crd.ClientGeneratorInterface,
 	executorTypes[ndm.GetTypeName(ctx)] = ndm
 	executorTypes[cnm.GetTypeName(ctx)] = cnm
 
+	// Pre-populate DefaultNSResolver with managed (labeled) namespaces so that
+	// AdoptExistingResources and CleanupOldExecutorObjects cover the full tenant NS set.
+	// Must run before the adopt/cleanup goroutines below. Non-fatal on error.
+	multitenant.PreRegisterManagedNamespaces(ctx, logger, kubernetesClient)
+
 	adoptExistingResources, _ := strconv.ParseBool(os.Getenv("ADOPT_EXISTING_RESOURCES"))
 
 	wg := &sync.WaitGroup{}