feat(mqtt): add MQTT listener, client creation, and shutdown wiring

Wire up the MQTT server-side orchestration layer (Task 1 of 7):
- Create NatsServer.Mqtt.cs with StartMqttListener(), CreateMqttClient(), MqttAddr()
- Forward MqttHandler.StartMqtt() to server.StartMqttListener()
- Add _mqttListener to Shutdown() doneExpected counting
- Fix ReadyForConnections to recognize active MQTT listener
- Handle RandomPort (-1) as ephemeral for MQTT listener
- Remove duplicate Mqtt field from ClientConnection.cs (already in ClientConnection.Mqtt.cs)
- Add 2 MQTT boot integration tests (accept + shutdown lifecycle)

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

@@ -79,9 +79,11 @@ public sealed partial class NatsServer
             _listener = null;
         }
         doneExpected += CloseWebsocketServer();
-        if (_gateway.Enabled)
+        if (_mqttListener != null)
         {
-            // mqtt listener managed by session 22
+            doneExpected++;
+            _mqttListener.Stop();
+            _mqttListener = null;
         }
         if (_leafNodeListener != null)
         {
@@ -505,7 +507,7 @@ public sealed partial class NatsServer
             leafOk    = opts.LeafNode.Port == 0 || _leafNodeListener != null;
             leafErr = _leafNodeListenerErr;
             wsOk = opts.Websocket.Port == 0 || _websocket.Listener != null;
-            mqttOk = opts.Mqtt.Port == 0;
+            mqttOk = opts.Mqtt.Port == 0 || _mqttListener != null;
             _mu.ExitReadLock();
 
             checks["server"] = (serverOk, serverErr);