Preserve nested masks with data-mask serialization - #20094

Description

This pull request makes Table.write(..., serialize_method="data_mask") propagate the selected mask serialization into compatible masked components nested inside mixin columns.

For a masked SkyCoord, this writes separate sc.ra.mask and sc.dec.mask columns, preserving both independently masked components and their underlying values. Selector matching remains at the original table-column level for string, column-name, and column-class forms. Nested serializers that do not support masked-data methods, such as a Time stored in SkyCoord.obstime, retain their existing serialization.

The regression uses disjoint coordinate masks and distinct underlying data, and also checks default behavior, a nonmatching selector, context restoration, and nested Time.

Fixes #18704

Validation

• Targeted nested-mask regression: 7 passed
• pytest -q astropy/table/tests astropy/io/ascii/tests/test_ecsv.py astropy/utils/masked/tests/test_table.py — 2,930 passed, 183 skipped, 14 xfailed
• pytest -q astropy/io/fits/tests/test_connect.py astropy/io/misc/tests/test_hdf5.py — 175 passed, 123 skipped
• Full pre-commit suite on the four changed files, including pinned Ruff check/format, codespell, repository review, and Sphinx lint
• git range-diff confirms that rebasing onto current main did not change either patch

AI assistance

I used OpenAI Codex and Anthropic Claude Code as development and review assistants, with human direction and review throughout. I have reviewed and understand the submitted implementation and tests, take responsibility for all submitted content and licensing compliance, and will engage directly with reviewer questions and feedback.

• By checking this box, the PR author has requested that maintainers do NOT use the "Squash and Merge" button.