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/MqttPacketWriterTests.cs

@@ -0,0 +1,20 @@
+using NATS.Server.Mqtt;
+
+namespace NATS.Server.Mqtt.Tests.Mqtt;
+
+public class MqttPacketWriterTests
+{
+    [Fact]
+    public void Writer_emits_fixed_header_and_round_trips_with_reader()
+    {
+        byte[] payload = Enumerable.Repeat((byte)0xAB, 130).ToArray();
+
+        var encoded = MqttPacketWriter.Write(MqttControlPacketType.Publish, payload);
+        encoded[0].ShouldBe((byte)0x30); // PUBLISH type with default flags
+
+        var decoded = MqttPacketReader.Read(encoded);
+        decoded.Type.ShouldBe(MqttControlPacketType.Publish);
+        decoded.RemainingLength.ShouldBe(payload.Length);
+        decoded.Payload.ToArray().ShouldBe(payload);
+    }
+}