feat: add pedantic subject validation and max payload enforcement on PUB

Move max payload validation from the parser to ProcessPubAsync so the
server sends -ERR 'Maximum Payload Violation' and closes the connection
(matching Go reference client.go:2442). In pedantic mode, reject PUB
with wildcard subjects via -ERR 'Invalid Publish Subject' (client.go:2869).
Add disposed guard to SubList.Remove to prevent crash during shutdown.

src/NATS.Server/Subscriptions/SubList.cs

@@ -15,8 +15,13 @@ public sealed class SubList : IDisposable
     private readonly TrieLevel _root = new();
     private Dictionary<string, SubListResult>? _cache = new(StringComparer.Ordinal);
     private uint _count;
+    private volatile bool _disposed;
 
-    public void Dispose() => _lock.Dispose();
+    public void Dispose()
+    {
+        _disposed = true;
+        _lock.Dispose();
+    }
 
     public uint Count
     {
@@ -95,6 +100,7 @@ public sealed class SubList : IDisposable
 
     public void Remove(Subscription sub)
     {
+        if (_disposed) return;
         _lock.EnterWriteLock();
         try
         {