Prefer valid distributions when collecting entry points

[gaoflow]

Fixes #534

Summary

• add a duplicate-aware distribution selector for entry_points() that replaces an invalid/empty dist-info entry with a later valid distribution of the same normalized name
• keep normal discovery order for unique distributions, avoiding the broader cost of preferring valid metadata for every distribution
• add a regression test where an empty older foo.dist-info appears before a valid one with entry points

Tests

• Red before the fix: PYTHONPATH=. uv run --python 3.11 --with '.[test]' pytest tests/test_main.py::InvalidMetadataTests::test_entry_points_prefer_valid_distribution -q failed with KeyError: 'main'\n- PYTHONPATH=. uv run --python 3.11 --with '.[test]' pytest tests/test_main.py::InvalidMetadataTests::test_entry_points_prefer_valid_distribution -q\n- PYTHONPATH=. uv run --python 3.11 --with '.[test]' pytest tests/test_main.py::InvalidMetadataTests tests/test_api.py::APITests::test_entry_points tests/test_api.py::APITests::test_entry_points_unique_packages_normalized -q\n- PYTHONPATH=. uv run --python 3.11 --with '.[test]' pytest -q\n- PYTHONPATH=. uv run --python 3.11 --with '.[test,check]' pytest --ruff -q\n- PYTHONPATH=. uv run --python 3.11 --with '.[test,type]' pytest --mypy -q\n- git diff --check\n\nAI assistance was used under my direction.