Add a test to check that GIL remains disabled under free-threaded Python.

Also add manual `Py_GIL_DISABLED` macro definition in CMakeLists as well, since there are some STATIC libraries that aren't getting it automatically through pybind11. As originally proposed in PR #3259.

Co-authored-by: Devansh Raulo <151062727+devansh0703@users.noreply.github.com>
PiperOrigin-RevId: 942216446
Change-Id: I9ef5f457491ae776c452912fbfa8617b8509d47c

python/mujoco/gil_test.py

@@ -0,0 +1,35 @@
+# Copyright 2026 DeepMind Technologies Limited
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+# ==============================================================================
+"""Tests for GIL-free operation under free-threaded Python."""
+
+import sys
+import sysconfig
+
+from absl.testing import absltest
+import mujoco  # pytype: disable=unused-import
+
+
+class GilDisabledTest(absltest.TestCase):
+
+  @absltest.skipUnless(
+      sysconfig.get_config_var('Py_GIL_DISABLED'),
+      'Not running under free-threaded Python',
+  )
+  def test_gil_is_disabled(self):
+    self.assertFalse(sys._is_gil_enabled())  # pylint: disable=protected-access
+
+
+if __name__ == '__main__':
+  absltest.main()