tests: add hyperlink and chart data label/legend roundtrip tests

[vs650vivek]

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.