a11y: RevealJS/Dashboard axe report scroll region lacks keyboard focus (scrollable-region-focusable)

Start in src/resources/formats/html/axe/axe-check.js, reading the existing overlay handling and the RevealJS and Dashboard branches linked in the issue. Check the generated report slide and #quarto-axe-offcanvas .offcanvas-body with the Playwright self-scan or axe-core. Done means both scrolling ancestors are keyboard reachable and no scrollable-region-focusable violations remain, with the outdated comment corrected.

Found while adding automated Playwright self-scan coverage for the axe accessibility-checker's own report chrome (it audits the page, then injects a report UI that never gets audited itself unless a test scans it directly).

axe rule: scrollable-region-focusable · impact: serious · tag: wcag2a (2.1.1, 2.1.3)
Targets: the RevealJS report slide (section.quarto-axe-report-slide) and the Dashboard offcanvas body (.offcanvas-body inside #quarto-axe-offcanvas).

When the report grows past its container's height, both the reveal report slide and the dashboard offcanvas body become scrollable (overflow-y: auto), but neither element nor any descendant is keyboard-focusable, so keyboard-only users can't reach the rest of the violations.

The HTML overlay variant already handles this case:

https://github.com/quarto-dev/quarto-cli/blob/13b7e598805cb63b385be5e9bc3fa96d8fd7bb3b/src/resources/formats/html/axe/axe-check.js#L346-L358

The comment there reasons the reveal/dashboard variants don't need the same tab stop because "they don't scroll the report element itself" — true of the inner .quarto-axe-report, but the scrolling ancestor (the reveal slide / .offcanvas-body) is a different element the comment doesn't account for:

https://github.com/quarto-dev/quarto-cli/blob/13b7e598805cb63b385be5e9bc3fa96d8fd7bb3b/src/resources/formats/html/axe/axe-check.js#L360-L387
https://github.com/quarto-dev/quarto-cli/blob/13b7e598805cb63b385be5e9bc3fa96d8fd7bb3b/src/resources/formats/html/axe/axe-check.js#L389-L419

Confirmed directly with axe-core on an overflowing report in both formats: reveal's report slide (scrollHeight 901 vs clientHeight 700) and dashboard's offcanvas body (scrollHeight 774 vs clientHeight 658 — this one overflows even with a small, default-sized report) both get flagged.

We could mirror the overlay's fix — tabIndex = 0 / role="region" / aria-label on the scrolling ancestor in each case (the reveal <section> and the offcanvas body <div>), and correct the now-inaccurate comment above.

Related to #14378 (same axe rule, different target — document code blocks rather than the checker's own report UI). Part of #8706.