feat: add flush-before-close for graceful client shutdown

src/NATS.Server/NatsClient.cs

@@ -563,6 +563,31 @@ public sealed class NatsClient : IDisposable
         _logger.LogDebug("Client {ClientId} connection closed: {CloseReason}", Id, reason);
     }
 
+    /// <summary>
+    /// Flushes pending data (unless skip-flush is set) and closes the connection.
+    /// </summary>
+    public async Task FlushAndCloseAsync(bool minimalFlush = false)
+    {
+        if (!ShouldSkipFlush)
+        {
+            try
+            {
+                using var flushCts = new CancellationTokenSource(minimalFlush
+                    ? TimeSpan.FromMilliseconds(100)
+                    : TimeSpan.FromSeconds(1));
+                await _stream.FlushAsync(flushCts.Token);
+            }
+            catch (Exception)
+            {
+                // Best effort flush — don't let it prevent close
+            }
+        }
+
+        try { _socket.Shutdown(SocketShutdown.Both); }
+        catch (SocketException) { }
+        catch (ObjectDisposedException) { }
+    }
+
     public void RemoveAllSubscriptions(SubList subList)
     {
         foreach (var sub in _subs.Values)

src/NATS.Server/NatsServer.cs

@@ -86,11 +86,14 @@ public sealed class NatsServer : IMessageRouter, ISubListAccess, IDisposable
         // Wait for accept loop to exit
         await _acceptLoopExited.Task.WaitAsync(TimeSpan.FromSeconds(5)).ConfigureAwait(ConfigureAwaitOptions.SuppressThrowing);
 
-        // Close all client connections
+        // Close all client connections — flush first, then mark closed
+        var flushTasks = new List<Task>();
         foreach (var client in _clients.Values)
         {
             client.MarkClosed(ClosedState.ServerShutdown);
+            flushTasks.Add(client.FlushAndCloseAsync(minimalFlush: true));
         }
+        await Task.WhenAll(flushTasks).WaitAsync(TimeSpan.FromSeconds(2)).ConfigureAwait(ConfigureAwaitOptions.SuppressThrowing);
 
         // Wait for active client tasks to drain (with timeout)
         if (Volatile.Read(ref _activeClientCount) > 0)