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

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)