Resolve OpenGL backend lazily

[devshahofficial]

Draft follow-up for #3366, based on the original lazy-backend import fix from @kevinzakka.

This keeps MUJOCO_GL validation at import time but defers importing the selected backend until a GLContext is constructed. That lets import mujoco work on systems that never render and do not have OpenGL libraries available.

Local validation:

• python3 -m py_compile python/mujoco/rendering/classic/gl_context.py python/mujoco/gl_context_test.py
• direct importlib/mocked validation for backend delegation, native AttributeError conversion, and backend ImportError conversion
• git diff --check

Could not run python/mujoco/gl_context_test.py directly in this local Python because absl-py is not installed in the base environment.

Opening as draft to avoid duplicating the existing approved PR unless maintainers want this rebased version.

[devshahofficial]

Quick CI read: the lone macOS ARM failure is in CMake/GoogleTest discovery for the unrelated engine_inverse_test (empty JSON while linking); the OpenGL diff is not on that path, and the rest of the matrix is green. I tried rerunning the failed job, but fork authors do not have permission on the upstream workflow. Could a maintainer kick that job once? If it reproduces, I’ll dig into it.