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

tests/NATS.Server.Tests/TlsServerTests.cs

@@ -0,0 +1,137 @@
+using System.Net;
+using System.Net.Security;
+using System.Net.Sockets;
+using System.Text;
+using Microsoft.Extensions.Logging.Abstractions;
+using NATS.Server;
+
+namespace NATS.Server.Tests;
+
+public class TlsServerTests : IAsyncLifetime
+{
+    private readonly NatsServer _server;
+    private readonly int _port;
+    private readonly CancellationTokenSource _cts = new();
+    private readonly string _certPath;
+    private readonly string _keyPath;
+
+    public TlsServerTests()
+    {
+        _port = GetFreePort();
+        (_certPath, _keyPath) = TlsHelperTests.GenerateTestCertFiles();
+        _server = new NatsServer(
+            new NatsOptions
+            {
+                Port = _port,
+                TlsCert = _certPath,
+                TlsKey = _keyPath,
+            },
+            NullLoggerFactory.Instance);
+    }
+
+    public async Task InitializeAsync()
+    {
+        _ = _server.StartAsync(_cts.Token);
+        await _server.WaitForReadyAsync();
+    }
+
+    public async Task DisposeAsync()
+    {
+        await _cts.CancelAsync();
+        _server.Dispose();
+        File.Delete(_certPath);
+        File.Delete(_keyPath);
+    }
+
+    [Fact]
+    public async Task Tls_client_connects_and_receives_info()
+    {
+        using var tcp = new TcpClient();
+        await tcp.ConnectAsync(IPAddress.Loopback, _port);
+        using var netStream = tcp.GetStream();
+
+        // Read INFO (sent before TLS upgrade in Mode 2)
+        var buf = new byte[4096];
+        var read = await netStream.ReadAsync(buf);
+        var info = Encoding.ASCII.GetString(buf, 0, read);
+        info.ShouldStartWith("INFO ");
+        info.ShouldContain("\"tls_required\":true");
+
+        // Upgrade to TLS
+        using var sslStream = new SslStream(netStream, false, (_, _, _, _) => true);
+        await sslStream.AuthenticateAsClientAsync("localhost");
+
+        // Send CONNECT + PING over TLS
+        await sslStream.WriteAsync("CONNECT {}\r\nPING\r\n"u8.ToArray());
+        await sslStream.FlushAsync();
+
+        // Read PONG
+        var pongBuf = new byte[256];
+        read = await sslStream.ReadAsync(pongBuf);
+        var pong = Encoding.ASCII.GetString(pongBuf, 0, read);
+        pong.ShouldContain("PONG");
+    }
+
+    [Fact]
+    public async Task Tls_pubsub_works_over_encrypted_connection()
+    {
+        using var tcp1 = new TcpClient();
+        await tcp1.ConnectAsync(IPAddress.Loopback, _port);
+        using var ssl1 = await UpgradeToTlsAsync(tcp1);
+
+        using var tcp2 = new TcpClient();
+        await tcp2.ConnectAsync(IPAddress.Loopback, _port);
+        using var ssl2 = await UpgradeToTlsAsync(tcp2);
+
+        // Sub on client 1
+        await ssl1.WriteAsync("CONNECT {}\r\nSUB test 1\r\nPING\r\n"u8.ToArray());
+        await ssl1.FlushAsync();
+
+        // Wait for PONG to confirm subscription is registered
+        var pongBuf = new byte[256];
+        var pongRead = await ssl1.ReadAsync(pongBuf);
+        var pongStr = Encoding.ASCII.GetString(pongBuf, 0, pongRead);
+        pongStr.ShouldContain("PONG");
+
+        // Pub on client 2
+        await ssl2.WriteAsync("CONNECT {}\r\nPUB test 5\r\nhello\r\nPING\r\n"u8.ToArray());
+        await ssl2.FlushAsync();
+
+        // Client 1 should receive MSG (may arrive across multiple TLS records)
+        var msg = await ReadUntilAsync(ssl1, "hello");
+        msg.ShouldContain("MSG test 1 5");
+        msg.ShouldContain("hello");
+    }
+
+    private static async Task<string> ReadUntilAsync(Stream stream, string expected, int timeoutMs = 5000)
+    {
+        using var cts = new CancellationTokenSource(timeoutMs);
+        var sb = new StringBuilder();
+        var buf = new byte[4096];
+        while (!sb.ToString().Contains(expected))
+        {
+            var n = await stream.ReadAsync(buf, cts.Token);
+            if (n == 0) break;
+            sb.Append(Encoding.ASCII.GetString(buf, 0, n));
+        }
+        return sb.ToString();
+    }
+
+    private static async Task<SslStream> UpgradeToTlsAsync(TcpClient tcp)
+    {
+        var netStream = tcp.GetStream();
+        var buf = new byte[4096];
+        _ = await netStream.ReadAsync(buf); // Read INFO (discard)
+
+        var ssl = new SslStream(netStream, false, (_, _, _, _) => true);
+        await ssl.AuthenticateAsClientAsync("localhost");
+        return ssl;
+    }
+
+    private static int GetFreePort()
+    {
+        using var sock = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
+        sock.Bind(new IPEndPoint(IPAddress.Loopback, 0));
+        return ((IPEndPoint)sock.LocalEndPoint!).Port;
+    }
+}