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/MqttPacketParserTests.cs

@@ -0,0 +1,26 @@
+using NATS.Server.Mqtt;
+
+namespace NATS.Server.Mqtt.Tests.Mqtt;
+
+public class MqttPacketParserTests
+{
+    [Fact]
+    public void Connect_packet_fixed_header_and_remaining_length_parse_correctly()
+    {
+        var packet = MqttPacketReader.Read(ConnectPacketBytes.Sample);
+        packet.Type.ShouldBe(MqttControlPacketType.Connect);
+        packet.RemainingLength.ShouldBe(12);
+        packet.Payload.Length.ShouldBe(12);
+    }
+
+    private static class ConnectPacketBytes
+    {
+        public static readonly byte[] Sample =
+        [
+            0x10, 0x0C, // CONNECT + remaining length
+            0x00, 0x04, (byte)'M', (byte)'Q', (byte)'T', (byte)'T',
+            0x04, 0x02, 0x00, 0x3C, // protocol level/flags/keepalive
+            0x00, 0x00, // empty client id
+        ];
+    }
+}