feat: enforce MaxConnections limit in accept loop

src/NATS.Server/NatsServer.cs

@@ -1,6 +1,7 @@
 using System.Collections.Concurrent;
 using System.Net;
 using System.Net.Sockets;
+using System.Text;
 using Microsoft.Extensions.Logging;
 using NATS.Server.Protocol;
 using NATS.Server.Subscriptions;
@@ -56,6 +57,32 @@ public sealed class NatsServer : IMessageRouter, ISubListAccess, IDisposable
             while (!ct.IsCancellationRequested)
             {
                 var socket = await _listener.AcceptAsync(ct);
+
+                // Check MaxConnections before creating the client
+                if (_options.MaxConnections > 0 && _clients.Count >= _options.MaxConnections)
+                {
+                    _logger.LogWarning("Client connection rejected: maximum connections ({MaxConnections}) exceeded",
+                        _options.MaxConnections);
+                    try
+                    {
+                        var stream = new NetworkStream(socket, ownsSocket: false);
+                        var errBytes = Encoding.ASCII.GetBytes(
+                            $"-ERR '{NatsProtocol.ErrMaxConnectionsExceeded}'\r\n");
+                        await stream.WriteAsync(errBytes, ct);
+                        await stream.FlushAsync(ct);
+                        stream.Dispose();
+                    }
+                    catch (Exception ex)
+                    {
+                        _logger.LogDebug(ex, "Failed to send -ERR to rejected client");
+                    }
+                    finally
+                    {
+                        socket.Dispose();
+                    }
+                    continue;
+                }
+
                 var clientId = Interlocked.Increment(ref _nextClientId);
 
                 _logger.LogDebug("Client {ClientId} connected from {RemoteEndpoint}", clientId, socket.RemoteEndPoint);