Reset marker config during unconfigure

[tobiasraabe]

Summary

Reset the global marker generator configuration when pytask unconfigures so process-local strict marker settings cannot affect later marker access.

Root Cause

pytask_parse_config() stores the active config on the singleton MARK_GEN. A run using --strict-markers left that config attached after pytask_unconfigure(), so later code in the same process could treat unknown markers as strict errors instead of warning.

Changes

• Add a marker plugin pytask_unconfigure() hook that clears MARK_GEN.config.
• Add a regression test that runs a strict-marker failure before accessing an unknown marker through pytask.mark.
• Document the fix in CHANGELOG.md.

Validation

uv run --group test pytest tests/test_mark.py::test_error_with_unknown_marker_and_strict tests/test_mark_structures.py::test_aliases tests/test_mark.py::test_strict_markers_do_not_leak_after_unconfigure -q