fix: [2.6] always call handleNodeUp in rewatchNodes for proper stopping balance (#45963)

Cherry-pick from master
pr: #45961 
Related to #45960

When QueryCoord restarts or reconnects to etcd, the rewatchNodes
function previously skipped handleNodeUp for QueryNodes in stopping
state. This caused stopping balance to fail because necessary components
were not initialized:
- Task scheduler executor was not added
- Dist handler was not started
- Node was not registered in resource manager

This fix ensures handleNodeUp is always called for new nodes regardless
of their stopping state, followed by handleNodeStopping if the node is
stopping. This allows the graceful shutdown process to correctly migrate
segments and channels away from stopping nodes.

---------

Signed-off-by: Congqi Xia <congqi.xia@zilliz.com>

internal/querycoordv2/meta/resource_manager.go

@@ -532,7 +532,12 @@ func (rm *ResourceManager) HandleNodeUp(ctx context.Context, node int64) {
 }
 
 func (rm *ResourceManager) handleNodeUp(ctx context.Context, node int64) {
-	if nodeInfo := rm.nodeMgr.Get(node); nodeInfo == nil || nodeInfo.IsEmbeddedQueryNodeInStreamingNode() {
+	nodeInfo := rm.nodeMgr.Get(node)
+	if nodeInfo == nil || nodeInfo.IsEmbeddedQueryNodeInStreamingNode() {
+		return
+	}
+	if nodeInfo.IsStoppingState() {
+		log.Warn("node is stopping, skip handle node up in resource manager", zap.Int64("node", node))
 		return
 	}
 	rm.incomingNode.Insert(node)

internal/querycoordv2/server.go

@@ -730,11 +730,13 @@ func (s *Server) rewatchNodes(sessions map[string]*sessionutil.Session) error {
 				Labels:   nodeSession.GetServerLabel(),
 			}))
 
+			// call handleNodeUp no matter what state new querynode is in
+			// all component need this op so that stopping balance could work correctly
+			s.handleNodeUp(nodeSession.GetServerID())
+
 			if nodeSession.Stopping {
 				s.nodeMgr.Stopping(nodeSession.ServerID)
 				s.handleNodeStopping(nodeSession.ServerID)
-			} else {
-				s.handleNodeUp(nodeSession.GetServerID())
 			}
 		}
 	}