Avoid changing semantics of mjsPlugin.name across XML sections.

The `extension` section used to discard the plugin name and write the instance name in `plugin.name`, while all of the other sections wrote the instance name in `plugin.instance_name` and optionally the plugin name in `plugin.name` (via the function `OnePlugin`). Now all plugin names are in `plugin.plugin_name` and the instance is written in `plugin.name`, so that it is possible to lookup a plugin via its unique instance name.

PiperOrigin-RevId: 683183804
Change-Id: I179ef5d221143b720db24d745b09e327bf1449ef

src/user/user_model.cc

@@ -311,7 +311,7 @@ void mjCModel::CopyPlugin(std::vector<mjCPlugin*>& dest,
 
   // only copy plugins that are referenced
   for (const auto& plugin : source) {
-    if (plugin->instance_name.empty() && plugin->model == this) {
+    if (plugin->name.empty() && plugin->model == this) {
       continue;
     }
     mjCPlugin* candidate = new mjCPlugin(*plugin);