feat: add close reason tracking to NatsClient

src/NATS.Server/NatsClient.cs

@@ -62,6 +62,12 @@ public sealed class NatsClient : IDisposable
     public long InBytes;
     public long OutBytes;
 
+    // Close reason tracking
+    private int _closeReason; // stores ClosedState as int for atomics
+    private int _skipFlushOnClose;
+    public ClosedState CloseReason => (ClosedState)Volatile.Read(ref _closeReason);
+    public bool ShouldSkipFlush => Volatile.Read(ref _skipFlushOnClose) != 0;
+
     // PING keepalive state
     private int _pingsOut;
     private long _lastIn;
@@ -139,13 +145,24 @@ public sealed class NatsClient : IDisposable
         catch (OperationCanceledException)
         {
             _logger.LogDebug("Client {ClientId} operation cancelled", Id);
+            MarkClosed(ClosedState.ServerShutdown);
+        }
+        catch (IOException)
+        {
+            MarkClosed(ClosedState.ReadError);
+        }
+        catch (SocketException)
+        {
+            MarkClosed(ClosedState.ReadError);
         }
         catch (Exception ex)
         {
             _logger.LogDebug(ex, "Client {ClientId} connection error", Id);
+            MarkClosed(ClosedState.ReadError);
         }
         finally
         {
+            MarkClosed(ClosedState.ClientClosed);
             try { _socket.Shutdown(SocketShutdown.Both); }
             catch (SocketException) { }
             catch (ObjectDisposedException) { }
@@ -522,6 +539,30 @@ public sealed class NatsClient : IDisposable
         }
     }
 
+    /// <summary>
+    /// Marks this connection as closed with the given reason.
+    /// Sets skip-flush flag for error-related reasons.
+    /// Thread-safe — only the first call sets the reason.
+    /// </summary>
+    public void MarkClosed(ClosedState reason)
+    {
+        if (Interlocked.CompareExchange(ref _closeReason, (int)reason, 0) != 0)
+            return;
+
+        switch (reason)
+        {
+            case ClosedState.ReadError:
+            case ClosedState.WriteError:
+            case ClosedState.SlowConsumerPendingBytes:
+            case ClosedState.SlowConsumerWriteDeadline:
+            case ClosedState.TLSHandshakeError:
+                Volatile.Write(ref _skipFlushOnClose, 1);
+                break;
+        }
+
+        _logger.LogDebug("Client {ClientId} connection closed: {CloseReason}", Id, reason);
+    }
+
     public void RemoveAllSubscriptions(SubList subList)
     {
         foreach (var sub in _subs.Values)