Prevent deletion from an attached mjSpec.

This change disallows calling `mjs_delete` on an mjSpec that has been attached to another mjSpec via `mjs_attach`. Attempting to delete an element from an attached spec will now result in an error. The Python bindings for `mjSpec.delete` have been updated to raise a ValueError when this occurs.

PiperOrigin-RevId: 897266695
Change-Id: Ic0670125a3028191ec50eca890f02b5910ec8b03

python/mujoco/specs_test.py

@@ -979,7 +979,6 @@ class SpecsTest(absltest.TestCase):
     self.assertEqual(mesh.plugin.name, 'inst')
     self.assertEqual(mesh.plugin.plugin_name, 'mujoco.sdf.torus')
 
-
   def test_duplicate_name_error(self):
     main_xml = """
     <mujoco>
@@ -1391,6 +1390,21 @@ class SpecsTest(absltest.TestCase):
     with self.assertRaisesRegex(ValueError, 'Frame not found.'):
       parent.attach(child4, frame='invalid_frame', prefix='child3-')
 
+  def test_delete_from_attached_spec_error(self):
+    parent = mujoco.MjSpec()
+    child = mujoco.MjSpec()
+    body = child.worldbody.add_body(name='child_body')
+    geom = body.add_geom(name='child_geom')
+
+    frame = parent.worldbody.add_frame()
+    parent.attach(child, frame=frame, prefix='child_')
+
+    # Now child spec is attached. Deleting from it should raise ValueError.
+    with self.assertRaisesRegex(
+        ValueError, 'Cannot delete element from an attached mjSpec.'
+    ):
+      child.delete(geom)
+
   def test_attach_valid_child_lists(self):
     xml1 = """
     <mujoco>