Adds targeted tests for hyperlink roundtrips (including target_slide) and chart label/legend behavior.

• tests/test_hyperlinks.py: verifies address/mailto/file-uri and ActionSetting.target_slide roundtrips.
• tests/test_chart_labels_legend.py: toggles data labels, number format, and asserts default legend/vary_by_categories for line chart.
• Updates pytest.ini to enforce coverage threshold (>=60%).
• Keeps new tests lint-clean.

…et coverage (+10%)

…-trip) — coverage uplift

test: fix pytest discovery; exclude helper dirs and quiet output

0942542

test(ci): add coverage threshold 60% via addopts

7f305a9

test: load unitutil fixtures; exclude only test_files from recursion

b06b05f

test: clean ruff issues and improve chart label roundtrip test

1b7d781

test: use ActionSetting.target_slide instead of Hyperlink.target_slide

45ab811

…_slide usage