xds/xdsdepmgr: skip unsubscribeFromCluster after Close

The unsubscribe closure returned by SubscribeToCluster is wrapped in
sync.OnceFunc and may run from the balancer tree Close after
DependencyManager.Close has already torn everything down and dropped
m.watcher. Before this change, the post-Close invocation reached
maybeSendUpdateLocked and panicked on m.watcher.Update.

Add an m.stopped guard at the top of unsubscribeFromCluster to match
the pattern used by every other post-Close callback in this file.
Restores the test/xds suite that started failing after 80dac1c.

Author: sushanb

internal/xds/xdsdepmgr/xds_dependency_manager.go

@@ -1005,6 +1005,13 @@ func (m *DependencyManager) SubscribeToCluster(name string) func() {
 func (m *DependencyManager) unsubscribeFromCluster(name string) {
 	m.mu.Lock()
 	defer m.mu.Unlock()
+	// Balancers can invoke the unsubscribe closure returned by
+	// SubscribeToCluster after Close has already torn down all watchers and
+	// dropped m.watcher / m.xdsClient. Match the m.stopped guard used by
+	// every other post-Close callback in this file.
+	if m.stopped {
+		return
+	}
 	c := m.clusterSubscriptions[name]
 	c.dynamicRefCount--
 	// This should not happen as unsubscribe returned from the