xds/xdsdepmgr: drop watcher and xdsClient references on Close

Per easwars' review: nil out m.watcher and m.xdsClient at the end of
DependencyManager.Close so the manager doesn't retain the resolver
(ConfigWatcher) or the xDS client after shutdown. Every callback that
touches those fields already re-checks m.stopped under m.mu, so the
nil assignments are safe once m.stopped is set.

Author: sushanb

internal/xds/xdsdepmgr/xds_dependency_manager.go

@@ -203,6 +203,12 @@ func (m *DependencyManager) Close() {
 		dnsResolver.stop()
 		delete(m.dnsResolvers, name)
 	}
+
+	// Drop references to externally-owned collaborators so the dependency
+	// manager itself doesn't retain them once Close returns. All in-flight
+	// callbacks re-check m.stopped under m.mu before touching these fields.
+	m.watcher = nil
+	m.xdsClient = nil
 }
 
 // annotateErrorWithNodeID annotates the given error with the provided xDS node