refactor: extract NATS.Server.Mqtt.Tests project

Move 29 MQTT test files from NATS.Server.Tests into a dedicated
NATS.Server.Mqtt.Tests project. Update namespaces, add
InternalsVisibleTo, and replace Task.Delay calls with
PollHelper.WaitUntilAsync for proper synchronization.

tests/NATS.Server.Mqtt.Tests/Mqtt/MqttKeepAliveTests.cs

@@ -0,0 +1,32 @@
+using System.Net;
+using System.Net.Sockets;
+using NATS.Server.Mqtt;
+using NATS.Server.TestUtilities;
+
+namespace NATS.Server.Mqtt.Tests.Mqtt;
+
+public class MqttKeepAliveTests
+{
+    [Fact]
+    public async Task Invalid_mqtt_credentials_or_keepalive_timeout_close_session_with_protocol_error()
+    {
+        await using var listener = new MqttListener("127.0.0.1", 0);
+        using var cts = new CancellationTokenSource();
+        await listener.StartAsync(cts.Token);
+
+        using var client = new TcpClient();
+        await client.ConnectAsync(IPAddress.Loopback, listener.Port);
+        var stream = client.GetStream();
+
+        await MqttRuntimeWire.WriteLineAsync(stream, "CONNECT keepalive-client keepalive=1");
+        (await MqttRuntimeWire.ReadLineAsync(stream, 1000)).ShouldBe("CONNACK");
+
+        // Poll until the server closes the connection due to keepalive expiry (keepalive=1s)
+        var disconnected = await PollHelper.WaitUntilAsync(async () =>
+        {
+            var result = await MqttRuntimeWire.ReadRawAsync(stream, 200);
+            return result == null;
+        }, timeoutMs: 5000, intervalMs: 100);
+        disconnected.ShouldBeTrue("Server should disconnect idle client after keepalive timeout");
+    }
+}