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; }
@@ -100,8 +101,13 @@ 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) =>
+        {
+            if (_callbacks.TryGetValue(s, out var cb))
             {
                 _receiveQueue.Writer.TryWrite(new InternalSystemMessage
                 {
@@ -112,8 +118,9 @@ public sealed class InternalEventSystem : IAsyncDisposable
                     Reply = reply,
                     Headers = hdr,
                     Message = msg,
-                Callback = callback,
+                    Callback = cb,
                 });
+            }
         };
 
         SystemAccount.SubList.Insert(sub);