feat(batch26): implement websocket frame/core feature group A

dotnet/src/ZB.MOM.NatsNet.Server/NatsServer.Lifecycle.cs

@@ -503,7 +503,7 @@ public sealed partial class NatsServer
             gatewayErr = _gatewayListenerErr;
             leafOk    = opts.LeafNode.Port == 0 || _leafNodeListener != null;
             leafErr = _leafNodeListenerErr;
-            wsOk = opts.Websocket.Port == 0;
+            wsOk = opts.Websocket.Port == 0 || _websocket.Listener != null;
             mqttOk = opts.Mqtt.Port == 0;
             _mu.ExitReadLock();
 
@@ -952,10 +952,10 @@ public sealed partial class NatsServer
     }
 
     /// <summary>
-    /// Stub — closes WebSocket server if running (session 23).
+    /// Closes the WebSocket server if running.
     /// Returns the number of done-channel signals to expect.
     /// </summary>
-    private int CloseWebsocketServer() => 0;
+    private int CloseWebsocketServer() => CloseWebsocketServerCore();
 
     /// <summary>
     /// Iterates over all route connections. Stub — session 14.