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/ClientConnection.cs

@@ -1803,7 +1803,7 @@ public sealed partial class ClientConnection
     // IsMqtt / IsWebSocket helpers (used by clientType, not separately tracked)
     // =========================================================================
 
-    internal bool IsMqtt()      => false; // Deferred to session 22 (MQTT).
+    internal bool IsMqtt()      => Mqtt != null;
     internal bool IsWebSocket() => Ws != null;
     internal bool IsHubLeafNode() => Kind == ClientKind.Leaf && Leaf?.IsSpoke != true;
     internal string RemoteCluster() => Leaf?.RemoteCluster ?? string.Empty;