feat: add new NatsOptions fields for Go config parity

Adds 10 new fields to NatsOptions (ClientAdvertise, TraceVerbose, MaxTracedMsgLen,
DisableSublistCache, ConnectErrorReports, ReconnectErrorReports, NoHeaderSupport,
MaxClosedClients, NoSystemAccount, SystemAccount) plus InCmdLine tracking set.
Moves MaxClosedClients from a private constant in NatsServer to a configurable option.

src/NATS.Server/NatsOptions.cs

@@ -71,6 +71,21 @@ public sealed class NatsOptions
     // Profiling (0 = disabled)
     public int ProfPort { get; set; }
 
+    // Extended options for Go parity
+    public string? ClientAdvertise { get; set; }
+    public bool TraceVerbose { get; set; }
+    public int MaxTracedMsgLen { get; set; }
+    public bool DisableSublistCache { get; set; }
+    public int ConnectErrorReports { get; set; } = 3600;
+    public int ReconnectErrorReports { get; set; } = 1;
+    public bool NoHeaderSupport { get; set; }
+    public int MaxClosedClients { get; set; } = 10_000;
+    public bool NoSystemAccount { get; set; }
+    public string? SystemAccount { get; set; }
+
+    // Tracks which fields were set via CLI flags (for reload precedence)
+    public HashSet<string> InCmdLine { get; } = [];
+
     // TLS
     public string? TlsCert { get; set; }
     public string? TlsKey { get; set; }

src/NATS.Server/NatsServer.cs

@@ -20,7 +20,6 @@ public sealed class NatsServer : IMessageRouter, ISubListAccess, IDisposable
     private readonly NatsOptions _options;
     private readonly ConcurrentDictionary<ulong, NatsClient> _clients = new();
     private readonly ConcurrentQueue<ClosedClient> _closedClients = new();
-    private const int MaxClosedClients = 10_000;
     private readonly ServerInfo _serverInfo;
     private readonly ILogger<NatsServer> _logger;
     private readonly ILoggerFactory _loggerFactory;
@@ -647,7 +646,7 @@ public sealed class NatsServer : IMessageRouter, ISubListAccess, IDisposable
         });
 
         // Cap closed clients list
-        while (_closedClients.Count > MaxClosedClients)
+        while (_closedClients.Count > _options.MaxClosedClients)
             _closedClients.TryDequeue(out _);
 
         var subList = client.Account?.SubList ?? _globalAccount.SubList;

tests/NATS.Server.Tests/NatsOptionsTests.cs

@@ -14,4 +14,20 @@ public class NatsOptionsTests
         opts.LogSizeLimit.ShouldBe(0L);
         opts.Tags.ShouldBeNull();
     }
+
+    [Fact]
+    public void New_fields_have_correct_defaults()
+    {
+        var opts = new NatsOptions();
+        opts.ClientAdvertise.ShouldBeNull();
+        opts.TraceVerbose.ShouldBeFalse();
+        opts.MaxTracedMsgLen.ShouldBe(0);
+        opts.DisableSublistCache.ShouldBeFalse();
+        opts.ConnectErrorReports.ShouldBe(3600);
+        opts.ReconnectErrorReports.ShouldBe(1);
+        opts.NoHeaderSupport.ShouldBeFalse();
+        opts.MaxClosedClients.ShouldBe(10_000);
+        opts.NoSystemAccount.ShouldBeFalse();
+        opts.SystemAccount.ShouldBeNull();
+    }
 }