Namespace plugin instances when attaching.

Fixes #2112.

PiperOrigin-RevId: 681045462
Change-Id: I2c945c5405fcb91e0f9a1b0b1e94f68d174c84ee

src/user/user_model.cc

@@ -341,6 +341,7 @@ mjCModel& mjCModel::operator+=(const mjCModel& other) {
     std::unordered_map<mjCPlugin*, mjCPlugin*> plugin_map;
     for (const auto& plugin : other.plugins_) {
       plugins_.push_back(new mjCPlugin(*plugin));
+      plugins_.back()->NameSpace(&other);
       plugin_map[plugin] = plugins_.back();
     }
     mapplugin(plugin_map, bodies_);