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/tests/ZB.MOM.NatsNet.Server.IntegrationTests/ServerBootTests.cs

@@ -88,6 +88,78 @@ public sealed class ServerBootTests : IDisposable
         }
     }
 
+    /// <summary>
+    /// Validates that a server can boot with an MQTT listener on an ephemeral port,
+    /// accept a TCP connection on the MQTT port, and register it as a client.
+    /// </summary>
+    [Fact]
+    public async Task MqttBoot_AcceptsConnection_ShouldSucceed()
+    {
+        var opts = new ServerOptions
+        {
+            Host = "127.0.0.1",
+            Port = 0,
+            Mqtt = { Port = -1, Host = "127.0.0.1" },
+        };
+
+        var (server, err) = NatsServer.NewServer(opts);
+        err.ShouldBeNull("NewServer should succeed");
+        server.ShouldNotBeNull();
+
+        try
+        {
+            server!.Start();
+
+            // Verify MQTT listener is up
+            var mqttAddr = server.MqttAddr();
+            mqttAddr.ShouldNotBeNull("MqttAddr should return the MQTT listener address");
+            mqttAddr!.Port.ShouldBeGreaterThan(0);
+
+            // ReadyForConnections should include MQTT
+            server.ReadyForConnections(TimeSpan.FromSeconds(5)).ShouldBeTrue();
+
+            // Connect a raw TCP client to the MQTT port
+            using var tcp = new System.Net.Sockets.TcpClient();
+            using var cts = new CancellationTokenSource(TimeSpan.FromSeconds(5));
+            await tcp.ConnectAsync(mqttAddr.Address, mqttAddr.Port, cts.Token);
+            tcp.Connected.ShouldBeTrue();
+
+            // Give CreateMqttClient a moment to register
+            await Task.Delay(100);
+            server.NumClients().ShouldBeGreaterThan(0);
+        }
+        finally
+        {
+            server!.Shutdown();
+        }
+    }
+
+    /// <summary>
+    /// Validates that an MQTT listener starts and shuts down cleanly.
+    /// </summary>
+    [Fact]
+    public void MqttBoot_StartAndShutdown_ShouldSucceed()
+    {
+        var opts = new ServerOptions
+        {
+            Host = "127.0.0.1",
+            Port = 0,
+            DontListen = true,
+            Mqtt = { Port = -1, Host = "127.0.0.1" },
+        };
+
+        var (server, err) = NatsServer.NewServer(opts);
+        err.ShouldBeNull();
+        server.ShouldNotBeNull();
+
+        server!.Start();
+        server.Running().ShouldBeTrue();
+        server.MqttAddr().ShouldNotBeNull();
+
+        server.Shutdown();
+        server.Running().ShouldBeFalse();
+    }
+
     /// <summary>
     /// Validates that Shutdown() after Start() completes cleanly.
     /// Uses DontListen to skip TCP binding — tests lifecycle only.