feat(mqtt): implement CONNECT/CONNACK/DISCONNECT packet handlers

Implement Task 3 of MQTT orchestration:
- Create MqttPacketHandlers.cs with ParseConnect(), ProcessConnect(), EnqueueConnAck(), HandleDisconnect()
- Wire CONNECT and DISCONNECT dispatch in MqttParser.cs
- Parse CONNECT: protocol name/level, flags, keep-alive, client ID, will, auth
- Send CONNACK (4-byte fixed packet with return code)
- DISCONNECT clears will message and closes connection cleanly
- Auto-generate client ID for empty ID + clean session
- Validate reserved bit, will flags, username/password consistency
- Add Reader field to MqttHandler for per-connection parsing
- 11 unit tests for CONNECT parsing and processing
- 1 end-to-end integration test: TCP → CONNECT → CONNACK over the wire

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

@@ -160,6 +160,96 @@ public sealed class ServerBootTests : IDisposable
         server.Running().ShouldBeFalse();
     }
 
+    /// <summary>
+    /// End-to-end: TCP connect → send MQTT CONNECT → receive CONNACK.
+    /// Validates the full MQTT handshake over the wire.
+    /// </summary>
+    [Fact]
+    public async Task MqttBoot_ConnectHandshake_ShouldReceiveConnAck()
+    {
+        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();
+        server.ShouldNotBeNull();
+
+        try
+        {
+            server!.Start();
+            var mqttAddr = server.MqttAddr();
+            mqttAddr.ShouldNotBeNull();
+
+            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);
+
+            var stream = tcp.GetStream();
+
+            // Build and send MQTT CONNECT packet.
+            var connectPacket = BuildMqttConnectPacket("integration-test");
+            await stream.WriteAsync(connectPacket, cts.Token);
+            await stream.FlushAsync(cts.Token);
+
+            // Read CONNACK response (4 bytes).
+            var response = new byte[4];
+            var totalRead = 0;
+            while (totalRead < 4)
+            {
+                var n = await stream.ReadAsync(response.AsMemory(totalRead, 4 - totalRead), cts.Token);
+                if (n == 0) break;
+                totalRead += n;
+            }
+
+            totalRead.ShouldBe(4, "Should receive 4-byte CONNACK");
+            response[0].ShouldBe((byte)0x20, "Packet type should be CONNACK");
+            response[1].ShouldBe((byte)0x02, "Remaining length should be 2");
+            response[3].ShouldBe((byte)0x00, "Return code should be Accepted (0)");
+        }
+        finally
+        {
+            server!.Shutdown();
+        }
+    }
+
+    /// <summary>Builds a minimal MQTT CONNECT packet.</summary>
+    private static byte[] BuildMqttConnectPacket(string clientId)
+    {
+        var payload = new List<byte>();
+        // Protocol name "MQTT"
+        payload.AddRange(new byte[] { 0x00, 0x04 });
+        payload.AddRange(System.Text.Encoding.UTF8.GetBytes("MQTT"));
+        // Protocol level 4
+        payload.Add(0x04);
+        // Flags: clean session
+        payload.Add(0x02);
+        // Keep alive: 60s
+        payload.AddRange(new byte[] { 0x00, 0x3C });
+        // Client ID
+        var cidBytes = System.Text.Encoding.UTF8.GetBytes(clientId);
+        payload.Add((byte)(cidBytes.Length >> 8));
+        payload.Add((byte)(cidBytes.Length & 0xFF));
+        payload.AddRange(cidBytes);
+
+        // Fixed header
+        var result = new List<byte>();
+        result.Add(0x10); // CONNECT type
+        var remLen = payload.Count;
+        do
+        {
+            var b = (byte)(remLen & 0x7F);
+            remLen >>= 7;
+            if (remLen > 0) b |= 0x80;
+            result.Add(b);
+        } while (remLen > 0);
+        result.AddRange(payload);
+        return result.ToArray();
+    }
+
     /// <summary>
     /// Validates that Shutdown() after Start() completes cleanly.
     /// Uses DontListen to skip TCP binding — tests lifecycle only.