fix: use per-SID callback dictionary in SysSubscribe to support multiple subscriptions

src/NATS.Server/Events/InternalEventSystem.cs

@@ -58,6 +58,7 @@ public sealed class InternalEventSystem : IAsyncDisposable
 
     private ulong _sequence;
     private int _subscriptionId;
+    private readonly ConcurrentDictionary<string, SystemMessageHandler> _callbacks = new();
 
     public Account SystemAccount { get; }
     public InternalClient SystemClient { get; }
@@ -100,20 +101,26 @@ public sealed class InternalEventSystem : IAsyncDisposable
             Client = SystemClient,
         };
 
-        // Wrap callback in noInlineCallback pattern: enqueue to receive loop
+        // Store callback keyed by SID so multiple subscriptions work
+        _callbacks[sid] = callback;
+
+        // Set a single routing callback on the system client that dispatches by SID
         SystemClient.MessageCallback = (subj, s, reply, hdr, msg) =>
         {
-            _receiveQueue.Writer.TryWrite(new InternalSystemMessage
+            if (_callbacks.TryGetValue(s, out var cb))
             {
-                Sub = sub,
-                Client = SystemClient,
-                Account = SystemAccount,
-                Subject = subj,
-                Reply = reply,
-                Headers = hdr,
-                Message = msg,
-                Callback = callback,
-            });
+                _receiveQueue.Writer.TryWrite(new InternalSystemMessage
+                {
+                    Sub = sub,
+                    Client = SystemClient,
+                    Account = SystemAccount,
+                    Subject = subj,
+                    Reply = reply,
+                    Headers = hdr,
+                    Message = msg,
+                    Callback = cb,
+                });
+            }
         };
 
         SystemAccount.SubList.Insert(sub);