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

python/mujoco/codegen/generate_spec_bindings.py

@@ -457,7 +457,7 @@ def generate_add() -> None:
                   throw pybind11::value_error("plugin.name should be a string.");
                 }
                 try {
-                  plugin.instance_name = input->instance_name;
+                  plugin.plugin_name = input->plugin_name;
                 } catch (const py::cast_error &e) {
                   throw pybind11::value_error("plugin.instance_name should be a string.");
                 }

python/mujoco/specs_test.py

@@ -205,14 +205,14 @@ class SpecsTest(absltest.TestCase):
 
     # Add plugin.
     plugin = spec.add_plugin(
-        name='name',
-        instance_name='instance',
+        name='instance_name',
+        plugin_name='mujoco.plugin',
         plugin_slot=7,
         active=True,
         info='info',
     )
-    self.assertEqual(plugin.name, 'name')
-    self.assertEqual(plugin.instance_name, 'instance')
+    self.assertEqual(plugin.name, 'instance_name')
+    self.assertEqual(plugin.plugin_name, 'mujoco.plugin')
     self.assertEqual(plugin.plugin_slot, 7)
     self.assertEqual(plugin.active, True)
     self.assertEqual(plugin.info, 'info')
@@ -227,8 +227,8 @@ class SpecsTest(absltest.TestCase):
 
     # Add a body with a plugin.
     body_with_plugin = spec.worldbody.add_body(plugin=plugin)
-    self.assertEqual(body_with_plugin.plugin.name, 'name')
-    self.assertEqual(body_with_plugin.plugin.instance_name, 'instance')
+    self.assertEqual(body_with_plugin.plugin.name, 'instance_name')
+    self.assertEqual(body_with_plugin.plugin.plugin_name, 'mujoco.plugin')
     self.assertEqual(body_with_plugin.plugin.plugin_slot, 7)
     self.assertEqual(body_with_plugin.plugin.active, True)
     self.assertEqual(body_with_plugin.plugin.info, 'info')
@@ -731,7 +731,7 @@ class SpecsTest(absltest.TestCase):
     self.assertIsNotNone(spec.worldbody)
 
     body = spec.worldbody.add_body()
-    body.plugin.name = 'mujoco.elasticity.cable'
+    body.plugin.plugin_name = 'mujoco.elasticity.cable'
     body.plugin.id = spec.add_plugin()
     body.plugin.active = True
     self.assertEqual(body.plugin.id, 0)