Add plugins list to mjSpec Python API.

Fixes #2061.

PiperOrigin-RevId: 677762985
Change-Id: I8e407373e7b505d9b0f1c023e97c5ea92169bbf6

introspect/functions.py

@@ -10270,6 +10270,22 @@ FUNCTIONS: Mapping[str, FunctionDecl] = dict([
          ),
          doc='Safely cast an element as mjsMaterial, or return NULL if the element is not an mjsMaterial.',  # pylint: disable=line-too-long
      )),
+    ('mjs_asPlugin',
+     FunctionDecl(
+         name='mjs_asPlugin',
+         return_type=PointerType(
+             inner_type=ValueType(name='mjsPlugin'),
+         ),
+         parameters=(
+             FunctionParameterDecl(
+                 name='element',
+                 type=PointerType(
+                     inner_type=ValueType(name='mjsElement'),
+                 ),
+             ),
+         ),
+         doc='Safely cast an element as mjsPlugin, or return NULL if the element is not an mjsPlugin.',  # pylint: disable=line-too-long
+     )),
     ('mjs_setBuffer',
      FunctionDecl(
          name='mjs_setBuffer',

introspect/structs.py

@@ -8565,7 +8565,7 @@ STRUCTS: Mapping[str, StructDecl] = dict([
          declname='struct mjsPlugin_',
          fields=(
              StructFieldDecl(
-                 name='instance',
+                 name='element',
                  type=PointerType(
                      inner_type=ValueType(name='mjsElement'),
                  ),