feat: add no-responders CONNECT validation and tests

Reject connections that send no_responders:true without headers:true,
since the 503 HMSG response requires header support. Add three tests:
connection rejection, acceptance with headers, and 503 delivery flow.

src/NATS.Server/NatsClient.cs

@@ -374,6 +374,15 @@ public sealed class NatsClient : IDisposable
             Account.AddClient(Id);
         }
 
+        // Validate no_responders requires headers
+        if (ClientOpts.NoResponders && !ClientOpts.Headers)
+        {
+            _logger.LogDebug("Client {ClientId} no_responders requires headers", Id);
+            await CloseWithReasonAsync(ClientClosedReason.NoRespondersRequiresHeaders,
+                NatsProtocol.ErrNoRespondersRequiresHeaders);
+            return;
+        }
+
         _flags.SetFlag(ClientFlags.ConnectReceived);
         _flags.SetFlag(ClientFlags.ConnectProcessFinished);
         _logger.LogDebug("CONNECT received from client {ClientId}, name={ClientName}", Id, ClientOpts?.Name);