feat: wire TLS negotiation into NatsServer accept loop

Integrate TLS support into the server's connection accept path:
- Add SslServerAuthenticationOptions and TlsRateLimiter fields to NatsServer
- Extract AcceptClientAsync method for TLS negotiation, rate limiting, and
  TLS state extraction (protocol version, cipher suite, peer certificate)
- Add InfoAlreadySent flag to NatsClient to skip redundant INFO when
  TlsConnectionWrapper already sent it during negotiation
- Add TlsServerTests verifying TLS connect+INFO and TLS pub/sub

src/NATS.Server/NatsServer.cs

@@ -1,11 +1,14 @@
 using System.Collections.Concurrent;
 using System.Net;
+using System.Net.Security;
 using System.Net.Sockets;
+using System.Security.Cryptography.X509Certificates;
 using System.Text;
 using Microsoft.Extensions.Logging;
 using NATS.Server.Monitoring;
 using NATS.Server.Protocol;
 using NATS.Server.Subscriptions;
+using NATS.Server.Tls;
 
 namespace NATS.Server;
 
@@ -19,6 +22,8 @@ public sealed class NatsServer : IMessageRouter, ISubListAccess, IDisposable
     private readonly ILoggerFactory _loggerFactory;
     private readonly ServerStats _stats = new();
     private readonly TaskCompletionSource _listeningStarted = new(TaskCreationOptions.RunContinuationsAsynchronously);
+    private readonly SslServerAuthenticationOptions? _sslOptions;
+    private readonly TlsRateLimiter? _tlsRateLimiter;
     private Socket? _listener;
     private MonitorServer? _monitorServer;
     private ulong _nextClientId;
@@ -48,6 +53,17 @@ public sealed class NatsServer : IMessageRouter, ISubListAccess, IDisposable
             Port = options.Port,
             MaxPayload = options.MaxPayload,
         };
+
+        if (options.HasTls)
+        {
+            _sslOptions = TlsHelper.BuildServerAuthOptions(options);
+            _serverInfo.TlsRequired = !options.AllowNonTls;
+            _serverInfo.TlsAvailable = options.AllowNonTls;
+            _serverInfo.TlsVerify = options.TlsVerify;
+
+            if (options.TlsRateLimit > 0)
+                _tlsRateLimiter = new TlsRateLimiter(options.TlsRateLimit);
+        }
     }
 
     public async Task StartAsync(CancellationToken ct)
@@ -105,13 +121,7 @@ public sealed class NatsServer : IMessageRouter, ISubListAccess, IDisposable
 
                 _logger.LogDebug("Client {ClientId} connected from {RemoteEndpoint}", clientId, socket.RemoteEndPoint);
 
-                var clientLogger = _loggerFactory.CreateLogger($"NATS.Server.NatsClient[{clientId}]");
-                var networkStream = new NetworkStream(socket, ownsSocket: false);
-                var client = new NatsClient(clientId, networkStream, socket, _options, _serverInfo, clientLogger, _stats);
-                client.Router = this;
-                _clients[clientId] = client;
-
-                _ = RunClientAsync(client, ct);
+                _ = AcceptClientAsync(socket, clientId, ct);
             }
         }
         catch (OperationCanceledException)
@@ -120,6 +130,49 @@ public sealed class NatsServer : IMessageRouter, ISubListAccess, IDisposable
         }
     }
 
+    private async Task AcceptClientAsync(Socket socket, ulong clientId, CancellationToken ct)
+    {
+        try
+        {
+            // Rate limit TLS handshakes
+            if (_tlsRateLimiter != null)
+                await _tlsRateLimiter.WaitAsync(ct);
+
+            var networkStream = new NetworkStream(socket, ownsSocket: false);
+
+            // TLS negotiation (no-op if not configured)
+            var (stream, infoAlreadySent) = await TlsConnectionWrapper.NegotiateAsync(
+                socket, networkStream, _options, _sslOptions, _serverInfo,
+                _loggerFactory.CreateLogger("NATS.Server.Tls"), ct);
+
+            // Extract TLS state
+            TlsConnectionState? tlsState = null;
+            if (stream is SslStream ssl)
+            {
+                tlsState = new TlsConnectionState(
+                    ssl.SslProtocol.ToString(),
+                    ssl.NegotiatedCipherSuite.ToString(),
+                    ssl.RemoteCertificate as X509Certificate2);
+            }
+
+            var clientLogger = _loggerFactory.CreateLogger($"NATS.Server.NatsClient[{clientId}]");
+            var client = new NatsClient(clientId, stream, socket, _options, _serverInfo,
+                clientLogger, _stats);
+            client.Router = this;
+            client.TlsState = tlsState;
+            client.InfoAlreadySent = infoAlreadySent;
+            _clients[clientId] = client;
+
+            await RunClientAsync(client, ct);
+        }
+        catch (Exception ex)
+        {
+            _logger.LogDebug(ex, "Failed to accept client {ClientId}", clientId);
+            try { socket.Shutdown(SocketShutdown.Both); } catch { }
+            socket.Dispose();
+        }
+    }
+
     private async Task RunClientAsync(NatsClient client, CancellationToken ct)
     {
         try
@@ -199,6 +252,7 @@ public sealed class NatsServer : IMessageRouter, ISubListAccess, IDisposable
     {
         if (_monitorServer != null)
             _monitorServer.DisposeAsync().AsTask().GetAwaiter().GetResult();
+        _tlsRateLimiter?.Dispose();
         _listener?.Dispose();
         foreach (var client in _clients.Values)
             client.Dispose();