test: cover annotation lookup helpers

[byee4]

Summary

• Add six self-contained unit tests for public gene, transcript, exon, and name lookup helpers.
• Clarify the tested helpers' docstrings without changing implementation.
• Add a GitHub Actions job that runs the unit tests on PRs to master.

Verification

• python -m pytest -q unit_tests: 6 passed locally (Python 3.10).
• python -m pytest -q: legacy tests cannot collect because their expected generated test/data/*.gtf.db fixtures are absent. The new unit tests do not require those fixtures.