feat: add per-account SubList isolation for message routing

Subscriptions and message routing now go through account-specific SubLists
instead of a single global SubList. Clients in different accounts cannot
see each other's messages. When no account is specified (or auth is not
configured), all clients share the global $G account.

src/NATS.Server/NatsServer.cs

@@ -13,7 +13,6 @@ public sealed class NatsServer : IMessageRouter, ISubListAccess, IDisposable
 {
     private readonly NatsOptions _options;
     private readonly ConcurrentDictionary<ulong, NatsClient> _clients = new();
-    private readonly SubList _subList = new();
     private readonly ServerInfo _serverInfo;
     private readonly ILogger<NatsServer> _logger;
     private readonly ILoggerFactory _loggerFactory;
@@ -24,7 +23,7 @@ public sealed class NatsServer : IMessageRouter, ISubListAccess, IDisposable
     private Socket? _listener;
     private ulong _nextClientId;
 
-    public SubList SubList => _subList;
+    public SubList SubList => _globalAccount.SubList;
 
     public Task WaitForReadyAsync() => _listeningStarted.Task;
 
@@ -148,7 +147,8 @@ public sealed class NatsServer : IMessageRouter, ISubListAccess, IDisposable
     public void ProcessMessage(string subject, string? replyTo, ReadOnlyMemory<byte> headers,
         ReadOnlyMemory<byte> payload, NatsClient sender)
     {
-        var result = _subList.Match(subject);
+        var subList = sender.Account?.SubList ?? _globalAccount.SubList;
+        var result = subList.Match(subject);
 
         // Deliver to plain subscribers
         foreach (var sub in result.PlainSubs)
@@ -205,7 +205,8 @@ public sealed class NatsServer : IMessageRouter, ISubListAccess, IDisposable
     {
         _clients.TryRemove(client.Id, out _);
         _logger.LogDebug("Removed client {ClientId}", client.Id);
-        client.RemoveAllSubscriptions(_subList);
+        var subList = client.Account?.SubList ?? _globalAccount.SubList;
+        client.RemoveAllSubscriptions(subList);
         client.Account?.RemoveClient(client.Id);
     }
 
@@ -214,7 +215,6 @@ public sealed class NatsServer : IMessageRouter, ISubListAccess, IDisposable
         _listener?.Dispose();
         foreach (var client in _clients.Values)
             client.Dispose();
-        _subList.Dispose();
         foreach (var account in _accounts.Values)
             account.Dispose();
     }