feat: implement mqtt packet-level parser and writer

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

@@ -0,0 +1,26 @@
+using NATS.Server.Mqtt;
+
+namespace NATS.Server.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
+        ];
+    }
+}

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

@@ -0,0 +1,20 @@
+using NATS.Server.Mqtt;
+
+namespace NATS.Server.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);
+    }
+}