feat: cache INFO serialization — build once at startup instead of per-connection

Avoids re-serializing the same ServerInfo JSON on every new connection. The
cache is rebuilt when the ephemeral port is resolved. Connections that carry a
per-connection nonce (NKey auth) continue to serialize individually so the nonce
is included correctly.

src/NATS.Server/NatsServer.cs

@@ -43,6 +43,8 @@ public sealed class NatsServer : IMessageRouter, ISubListAccess, IDisposable
 
     private int _lameDuck;
 
+    private byte[] _cachedInfoLine = [];
+
     private readonly List<PosixSignalRegistration> _signalRegistrations = [];
 
     private string? _portsFilePath;
@@ -51,6 +53,7 @@ public sealed class NatsServer : IMessageRouter, ISubListAccess, IDisposable
     private static readonly TimeSpan AcceptMaxSleep = TimeSpan.FromSeconds(1);
 
     public SubList SubList => _globalAccount.SubList;
+    public byte[] CachedInfoLine => _cachedInfoLine;
     public ServerStats Stats => _stats;
     public DateTime StartTime => new(Interlocked.Read(ref _startTimeTicks), DateTimeKind.Utc);
     public string ServerId => _serverInfo.ServerId;
@@ -272,6 +275,14 @@ public sealed class NatsServer : IMessageRouter, ISubListAccess, IDisposable
             if (options.TlsRateLimit > 0)
                 _tlsRateLimiter = new TlsRateLimiter(options.TlsRateLimit);
         }
+
+        BuildCachedInfo();
+    }
+
+    private void BuildCachedInfo()
+    {
+        var infoJson = System.Text.Json.JsonSerializer.Serialize(_serverInfo);
+        _cachedInfoLine = Encoding.ASCII.GetBytes($"INFO {infoJson}\r\n");
     }
 
     public async Task StartAsync(CancellationToken ct)
@@ -292,6 +303,7 @@ public sealed class NatsServer : IMessageRouter, ISubListAccess, IDisposable
             var actualPort = ((IPEndPoint)_listener.LocalEndPoint!).Port;
             _options.Port = actualPort;
             _serverInfo.Port = actualPort;
+            BuildCachedInfo();
         }
 
         _listeningStarted.TrySetResult();