build: advertise LICENSE in sdist metadata

[Haricharanpanjwani]

Closes #755.

Summary

The sdist already ships the combined LICENSE file, but project.license-files only advertised LICENSE-wheel, NOTICE, and DISCLAIMER. That meant the generated PKG-INFO License-File metadata under-reported the combined Apache 2.0 + bundled MIT attribution file used for the website UI components.

This change adds LICENSE to project.license-files and adds a regression test to keep that metadata list from drifting.

Changes

• add LICENSE to project.license-files in pyproject.toml
• add a focused release-config test that asserts the expected license-files list

Validation

• python3 -m py_compile tests/test_release_config.py
• XDG_CACHE_HOME=/tmp/xdg-cache UV_CACHE_DIR=/tmp/uv-cache uv run --with pytest pytest tests/test_release_config.py -q
  • result: 2 passed, 1 warning in 0.01s
• XDG_CACHE_HOME=/tmp/xdg-cache UV_CACHE_DIR=/tmp/uv-cache uv run python -m flit build --format sdist
  • result: built dist/apache_burr-0.42.0.tar.gz
• tar -xOf dist/apache_burr-0.42.0.tar.gz 'apache_burr-0.42.0/PKG-INFO' | rg '^License-File:'\n - result includes DISCLAIMER, LICENSE, LICENSE-wheel, and NOTICE\n\n## Notes\n\n- The pytest run emitted an existing PytestConfigWarning about unknown config option asyncio_mode; it did not affect the test result.