Fix memory leak from undisposed sockets and event handler retention during server restart (#3608)

* Fix memory leak from undisposed sockets and retained event handlers during server restart

When an OPC UA server is stopped and restarted in-process, TcpServerChannel
instances were never garbage collected because UaSCBinaryChannel.Dispose() did
not close the underlying socket. Pending ReceiveAsync operations kept the
channels pinned via SocketAsyncEngine, causing a steady heap growth per cycle.

Changes:
- Close socket in UaSCBinaryChannel.Dispose() to cancel pending async I/O
- Unsubscribe CertificateValidator.CertificateUpdate in StandardServer.Dispose()
- Unsubscribe ConfigurationWatcher.Changed in StandardServer.Dispose()
- Unsubscribe ConnectionStatusChanged from TransportListeners in ServerBase.Dispose()

Verified over 786 restart cycles with stable heap.

* Remove verbose comment

* Remove redundant event unsubscribe before dispose

* Remove duplicate CertificateUpdate unsubscribe (already existed in master)

Author: RicoSuter

Stack/Opc.Ua.Core/Stack/Server/ServerBase.cs

@@ -81,6 +81,7 @@ protected virtual void Dispose(bool disposing)
                 {
                     for (int ii = 0; ii < TransportListeners.Count; ii++)
                     {
+                        TransportListeners[ii].ConnectionStatusChanged -= OnConnectionStatusChanged;
                         Utils.SilentDispose(TransportListeners[ii]);
                     }
 

Stack/Opc.Ua.Core/Stack/Tcp/UaSCBinaryChannel.cs

@@ -225,6 +225,7 @@ protected virtual void Dispose(bool disposing)
         {
             if (disposing)
             {
+                Socket?.Close();
                 DiscardTokens();
                 Utils.SilentDispose(Socket);