Add aria-sort to sorted DataTable column headers

[olitreadwell]

Problem

DataTable column headers did not expose the current sort state to
assistive technology. There was no aria-sort on any <th>, so a
screen reader user could not tell which column the table was sorted by,
or in which direction.

This is a distinct, self-found gap in the sortable headers. It is
related to the maintainer-confirmed screen-reader/table-semantics
discussion in #11108, but that issue is about clickable rows, not the
column-header sort state — so this PR does not close it.

Solution

Pass the active sort direction to the MUI TableCell through its
sortDirection prop. MUI maps that prop to aria-sort="ascending" or
aria-sort="descending" on the <th> element.

Only the currently sorted column receives aria-sort; other columns get
none, which matches the ARIA spec. The change is one prop on
DataTableHeadCell; no visual change.

How To Test

From the repo root:

yarn install
LANG=en_US.UTF-8 NODE_ENV=test BABEL_ENV=cjs NODE_ICU_DATA=./node_modules/full-icu \
  node_modules/.bin/jest packages/ra-ui-materialui/src/list/datatable/DataTable.spec.tsx

The three new tests under DataTable > Sorting fail on master and pass
with this change. Manual: open any DataTable, turn on a screen reader,
and confirm the sorted column announces "sorted ascending/descending".

Additional Checks

• The PR targets master for a bugfix or a documentation fix, or next for a feature
• The PR includes unit tests (3 new tests in DataTable.spec.tsx)
• The PR includes one or several stories — not needed: this is an a11y fix to existing behaviour, covered by the existing Basic DataTable story the new tests render.
• The documentation is up to date — no doc change: aria-sort is internal behaviour with no public API change.

Checks run locally

• Tests: PASS — DataTable.spec.tsx 30 passed (3 new); list/datatable folder 34 passed
• Lint (eslint): PASS on the two changed files (+ husky lint-staged on commit)
• Prettier: PASS on the two changed files
• Build/typecheck (lerna run build for ra-ui-materialui + deps): PASS

Scope

One logical change: aria-sort on the modern DataTable header cell + its
tests. The legacy Datagrid (DatagridHeaderCell) has the same gap; left
for a separate follow-up PR to keep this one reviewable.

AI disclosure

Drafted with AI assistance. I found the gap, wrote the failing tests
first to prove it, verified the fix makes them pass, and reviewed the
diff line by line. marmelab/react-admin does not require an AI-disclosure;
included here as good practice.

Promotion note (fork-staged)

This PR is staged inside the fork (olitreadwell/react-admin, base
master). To open it upstream:

gh pr create --repo marmelab/react-admin --base master \
  --head olitreadwell:contrib/marmelab-react-admin-20260805-110228

No CLA, DCO, or signed commits are required by CONTRIBUTING. Consider
referencing #11108 (related table-a11y discussion) when promoting.