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/src/ZB.MOM.NatsNet.Server/Mqtt/MqttParser.cs

@@ -126,18 +126,32 @@ internal static class MqttParser
                 case MqttPacket.Connect:
                     if (connected)
                     {
-                        // Second CONNECT on same connection is a protocol violation.
                         err = new InvalidOperationException("second CONNECT packet not allowed");
                         break;
                     }
-                    // TODO: Task 3 — MqttParseConnect + MqttProcessConnect
-                    err = new NotImplementedException("CONNECT not yet implemented");
+                    var (rc, cp, parseErr) = MqttPacketHandlers.ParseConnect(r);
+                    if (parseErr != null)
+                    {
+                        // Send CONNACK with error code if we have one, then close.
+                        if (rc != MqttConnAckRc.Accepted)
+                            MqttPacketHandlers.EnqueueConnAck(c, rc, false);
+                        err = parseErr;
+                        break;
+                    }
+                    if (rc != MqttConnAckRc.Accepted)
+                    {
+                        MqttPacketHandlers.EnqueueConnAck(c, rc, false);
+                        err = new InvalidOperationException($"CONNECT rejected with code 0x{rc:X2}");
+                        break;
+                    }
+                    err = MqttPacketHandlers.ProcessConnect(c, cp!);
+                    if (err == null)
+                        connected = true;
                     break;
 
                 case MqttPacket.Disconnect:
-                    // TODO: Task 3 — handle DISCONNECT
-                    err = new NotImplementedException("DISCONNECT not yet implemented");
-                    break;
+                    MqttPacketHandlers.HandleDisconnect(c);
+                    return null; // Connection closed, exit parse loop.
 
                 default:
                     err = new InvalidOperationException($"unknown MQTT packet type: 0x{pt:X2}");