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/NatsClient.cs

@@ -260,6 +260,13 @@ public sealed class NatsClient : IDisposable
             _logger.LogDebug("Client {ClientId} authenticated as {Identity}", Id, result.Identity);
         }
 
+        // If no account was assigned by auth, assign global account
+        if (Account == null && Router is NatsServer server2)
+        {
+            Account = server2.GetOrCreateAccount(Account.GlobalAccountName);
+            Account.AddClient(Id);
+        }
+
         ConnectReceived = true;
         _logger.LogDebug("CONNECT received from client {ClientId}, name={ClientName}", Id, ClientOpts?.Name);
     }
@@ -286,8 +293,7 @@ public sealed class NatsClient : IDisposable
 
         _logger.LogDebug("SUB {Subject} {Sid} from client {ClientId}", cmd.Subject, cmd.Sid, Id);
 
-        if (Router is ISubListAccess sl)
-            sl.SubList.Insert(sub);
+        Account?.SubList.Insert(sub);
     }
 
     private void ProcessUnsub(ParsedCommand cmd)
@@ -306,8 +312,7 @@ public sealed class NatsClient : IDisposable
 
         _subs.Remove(cmd.Sid!);
 
-        if (Router is ISubListAccess sl)
-            sl.SubList.Remove(sub);
+        Account?.SubList.Remove(sub);
     }
 
     private async ValueTask ProcessPubAsync(ParsedCommand cmd)
@@ -488,6 +493,7 @@ public sealed class NatsClient : IDisposable
 
     public void Dispose()
     {
+        _permissions?.Dispose();
         _clientCts?.Dispose();
         _stream.Dispose();
         _socket.Dispose();

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();
     }