Summary

Prevent sys_modules_saved() from removing a module while another thread is still importing it. The context now holds Python's import lock through the save/restore window, with a regression test covering the interleaving.

Fixes #2247

Verification

• Ran the issue's deterministic reproducer against current main; it fails before the fix and prints no failure with the branch.
• uv run --with-editable . --with-requirements requirements/pytest.pip --with-requirements requirements/pip.pip pytest tests/test_misc.py -q
• uv run --with ruff ruff format --check coverage/misc.py tests/test_misc.py
• git diff --check