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

include/mujoco/mjspec.h

@@ -183,7 +183,7 @@ typedef struct mjsOrientation_ {   // alternative orientation specifiers
 typedef struct mjsPlugin_ {        // plugin specification
   mjsElement* element;             // element type
   mjString* name;                  // name
-  mjString* instance_name;         // instance name
+  mjString* plugin_name;           // plugin name
   int plugin_slot;                 // global registered slot number of the plugin
   mjtByte active;                  // is the plugin active
   mjString* info;                  // message appended to compiler errors