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.Tests/Mqtt/MqttParserTests.cs

@@ -56,16 +56,39 @@ public sealed class MqttParserTests
     }
 
     [Fact]
-    public void Parse_ConnectFirst_ShouldNotRejectAsNonConnect()
+    public void Parse_ConnectFirst_ShouldAcceptConnect()
     {
         var c = CreateMqttClient();
-        // CONNECT packet (minimal): type=0x10, remaining len=0
-        // This will hit the "not yet implemented" but NOT the "first packet" error.
-        var buf = new byte[] { MqttPacket.Connect, 0x00 };
-        var err = MqttParser.Parse(c, buf, buf.Length);
-        err.ShouldNotBeNull(); // Will be NotImplementedException
-        err.ShouldBeOfType<NotImplementedException>();
-        err.Message.ShouldContain("CONNECT not yet implemented");
+        // Use a MemoryStream so CONNACK can be written.
+        typeof(ClientConnection)
+            .GetField("_nc", System.Reflection.BindingFlags.NonPublic | System.Reflection.BindingFlags.Instance)!
+            .SetValue(c, new MemoryStream());
+
+        // Build a valid CONNECT packet.
+        var payload = new List<byte>();
+        payload.AddRange(new byte[] { 0x00, 0x04 }); // protocol name length
+        payload.AddRange(System.Text.Encoding.UTF8.GetBytes("MQTT"));
+        payload.Add(0x04); // level
+        payload.Add(0x02); // flags: clean session
+        payload.AddRange(new byte[] { 0x00, 0x3C }); // keep alive = 60
+        payload.AddRange(new byte[] { 0x00, 0x04 }); // client id length
+        payload.AddRange(System.Text.Encoding.UTF8.GetBytes("test"));
+
+        var buf = new List<byte> { MqttPacket.Connect };
+        // Remaining length
+        var remLen = payload.Count;
+        do
+        {
+            var b = (byte)(remLen & 0x7F);
+            remLen >>= 7;
+            if (remLen > 0) b |= 0x80;
+            buf.Add(b);
+        } while (remLen > 0);
+        buf.AddRange(payload);
+
+        var err = MqttParser.Parse(c, buf.ToArray(), buf.Count);
+        err.ShouldBeNull("CONNECT should be accepted, not rejected as non-CONNECT");
+        (c.Flags & ClientFlags.ConnectReceived).ShouldNotBe((ClientFlags)0);
     }
 
     [Fact]