fix(datatable): improve sortable header accessibility

[edschema]

Description

This PR improves DataTable sortable header accessibility.

Keyboard operation; Sortable headers now render a native button inside the existing <th>, with React Table sort toggle props on the button. This makes sortable headers reachable with Tab and operable with Enter and Space.

Sort state: Sorted headers now expose sort direction with aria-sort on the primary sorted header only. Inactive sortable headers and secondary multi-sort headers omit aria-sort.

Column scope declaration: DataTable header cells also render scope="col", and scoped DataTable styles keep the sort button aligned with the existing header layout and click target.

Out of Scope

• Table captions, table labels, and explanatory text for sortable columns.
• The test output includes a pre-existing React warning about spreading a key prop. DataTable uses helpers that return key with other props; React wants key passed as its own JSX attribute. This PR does not change the existing pattern because fixing would increase the scope to touch header, row, and cell code throughout DataTable. I think cleaning up the key warning belongs in a separate PR; this PR is scoped to just the a11y fixes.
• The tests also show pre-existing React defaultProps warnings. These are not introduced by this PR and are out of scope.

Change Log

• Render sortable DataTable headers as native buttons.
• Move React Table sort toggle props from the <th> to the sort button.
• Add aria-sort for the primary sorted column and omit it for inactive or secondary sorted columns.
• Add scope="col" to DataTable header cells.
• Add DataTable styles for sortable header button layout and click area.
• Add tests for sort state, column scope, sortable button behavior, keyboard operation, DataTable sorting, and multi-sort aria-sort behavior.

Notes

• Visual sort indicators still render for sorted columns, including secondary multi-sort columns. Only the primary sorted column exposes aria-sort.
• The React key spread warning is existing behavior from React Table prop getters and is unchanged by this PR.
• Accessibility review is recommended because this changes table header semantics and keyboard behavior.

Tests Done

• npm test -- --runTestsByPath src/DataTable/tests/DataTable.test.jsx src/DataTable/tests/TableHeaderCell.test.jsx --coverage=false
• git diff --check

Deploy Preview

https://deploy-preview-4345--paragon-openedx-v23.netlify.app/components/datatable/#frontend-filtering-and-sorting

Merge Checklist

• If your update includes visual changes, have they been reviewed by a designer? Send them a link to the Netlify deploy preview, if applicable.
  • No intended visual change; Netlify deploy preview review is pending.
• Does your change adhere to the documented style conventions?
  • Added scoped pgn__ DataTable classes under the existing .pgn__data-table styles.
• Do any prop types have missing descriptions in the Props API tables in the documentation site (check deploy preview)?
  • No documented public props were added.
• Were your changes tested using all available themes (see theme switcher in the header of the deploy preview, under the "Settings" icon)?
  • Not run.
• Were your changes tested in the example app?
  • Not run.
• Is there adequate test coverage for your changes?
  • Focused TableHeaderCell tests and DataTable regression tests cover the changed behavior.
• Consider whether this change needs to reviewed/QA'ed for accessibility (a11y). If so, please request an a11y review for the PR in the #wg-paragon Open edX Slack channel.
  • Accessibility review is recommended because this changes table header semantics and keyboard behavior.

Post-merge Checklist

• Verify your changes were released to NPM at the expected version.
• If useful, share the contribution in #show-and-tell.

AI Assistance

AI tools were used to aid in diagnosis, planning, implementation, and review. All work was checked and reviewed.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.26%. Comparing base (9340b9c) to head (f9ad9ac).
⚠️ Report is 13 commits behind head on release-23.x.

Additional details and impacted files

@@               Coverage Diff                @@
##           release-23.x    #4345      +/-   ##
================================================
- Coverage         94.40%   94.26%   -0.14%     
================================================
  Files               242      242              
  Lines              4309     4309              
  Branches            981     1038      +57     
================================================
- Hits               4068     4062       -6     
- Misses              237      238       +1     
- Partials              4        9       +5     

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