feat: add -ERR response infrastructure with SendErrAsync and SendErrAndCloseAsync

src/NATS.Server/NatsClient.cs

@@ -29,6 +29,7 @@ public sealed class NatsClient : IDisposable
     private readonly ServerInfo _serverInfo;
     private readonly NatsParser _parser;
     private readonly SemaphoreSlim _writeLock = new(1, 1);
+    private CancellationTokenSource? _clientCts;
     private readonly Dictionary<string, Subscription> _subs = new();
     private readonly ILogger _logger;
 
@@ -58,15 +59,16 @@ public sealed class NatsClient : IDisposable
 
     public async Task RunAsync(CancellationToken ct)
     {
+        _clientCts = CancellationTokenSource.CreateLinkedTokenSource(ct);
         var pipe = new Pipe();
         try
         {
             // Send INFO
-            await SendInfoAsync(ct);
+            await SendInfoAsync(_clientCts.Token);
 
             // Start read pump and command processing in parallel
-            var fillTask = FillPipeAsync(pipe.Writer, ct);
-            var processTask = ProcessCommandsAsync(pipe.Reader, ct);
+            var fillTask = FillPipeAsync(pipe.Writer, _clientCts.Token);
+            var processTask = ProcessCommandsAsync(pipe.Reader, _clientCts.Token);
 
             await Task.WhenAny(fillTask, processTask);
         }
@@ -81,6 +83,14 @@ public sealed class NatsClient : IDisposable
         finally
         {
             Router?.RemoveClient(this);
+            try
+            {
+                _socket.Shutdown(SocketShutdown.Both);
+            }
+            catch (Exception ex)
+            {
+                _logger.LogDebug(ex, "Client {ClientId} socket shutdown error", Id);
+            }
         }
     }
 
@@ -282,6 +292,36 @@ public sealed class NatsClient : IDisposable
         }
     }
 
+    public async Task SendErrAsync(string message)
+    {
+        var errLine = Encoding.ASCII.GetBytes($"-ERR '{message}'\r\n");
+        try
+        {
+            await WriteAsync(errLine, _clientCts?.Token ?? CancellationToken.None);
+        }
+        catch (OperationCanceledException)
+        {
+            // Expected during shutdown
+        }
+        catch (IOException ex)
+        {
+            _logger.LogDebug(ex, "Client {ClientId} failed to send -ERR", Id);
+        }
+        catch (ObjectDisposedException ex)
+        {
+            _logger.LogDebug(ex, "Client {ClientId} failed to send -ERR (disposed)", Id);
+        }
+    }
+
+    public async Task SendErrAndCloseAsync(string message)
+    {
+        await SendErrAsync(message);
+        if (_clientCts is { } cts)
+            await cts.CancelAsync();
+        else
+            _socket.Close();
+    }
+
     public void RemoveAllSubscriptions(SubList subList)
     {
         foreach (var sub in _subs.Values)
@@ -291,6 +331,7 @@ public sealed class NatsClient : IDisposable
 
     public void Dispose()
     {
+        _clientCts?.Dispose();
         _stream.Dispose();
         _socket.Dispose();
         _writeLock.Dispose();

src/NATS.Server/Protocol/NatsProtocol.cs

@@ -19,6 +19,13 @@ public static class NatsProtocol
     public static readonly byte[] MsgPrefix = "MSG "u8.ToArray();
     public static readonly byte[] HmsgPrefix = "HMSG "u8.ToArray();
     public static readonly byte[] ErrPrefix = "-ERR "u8.ToArray();
+
+    // Standard error messages (matching Go server)
+    public const string ErrMaxConnectionsExceeded = "maximum connections exceeded";
+    public const string ErrStaleConnection = "Stale Connection";
+    public const string ErrMaxPayloadViolation = "Maximum Payload Violation";
+    public const string ErrInvalidPublishSubject = "Invalid Publish Subject";
+    public const string ErrInvalidSubject = "Invalid Subject";
 }
 
 public sealed class ServerInfo