Fix revealjs diagram and image announcements

[jooyoungseo]

Description

Fixes #15033.

Backport image announcements to the bundled reveal.js 5.1.0 without upgrading the presentation framework:

• Skip style and script contents, including Mermaid's inline SVG stylesheet.
• Announce native image/video alternative text, so fig-alt is included alongside the caption.
• Announce role="img" elements by their name rather than their presentational descendants, using the fix proposed in Fix screen reader image names and exclude SVG styles hakimel/reveal.js#3949.
• Keep the existing hidden-content filtering, 5.1.0 whitespace behavior, and SVG titles outside named images.

The readable source patch is included with an updater step that reapplies it and rebuilds the UMD/ESM bundles and source maps. The updater notes that the upstream PR is not yet merged and that both fixes must be present in a future reveal.js release before removing the backport.

This does not include the separate slide/fragment announcement changes in #15020 or change Graphviz's per-node title/label duplication.

Validation

• Rendered the new fixture with Quarto 1.10.19 and the regenerated reveal.js 5.1.0 bundles.
• Playwright: 27/27 tests pass across Chromium, Firefox, and WebKit (revealjs-image-announcements.spec.ts --workers=1).
• Before the fix, 8 of the 9 Chromium cases fail. WebKit's existing SVG-title filtering was also checked against the original bundle and is preserved.
• Applied the source patch to pristine 5.1.0 and verified that the result matches the build source after normalizing Windows line endings.
• Both bundle formats and maps were rebuilt using the upstream lockfile. The source-map source list is unchanged; only js/reveal.js differs.
• The full Quarto test suite was not run.

Checklist

I have (if applicable):

• referenced the GitHub issue this PR closes
• updated the appropriate changelog in the PR
• ensured the present test suite passes (focused browser tests pass; full suite not run)
• added new tests
• created a separate documentation PR in Quarto's website repo and linked it to this PR (not applicable to this bug fix)

AI-assisted PR

• AI tool used: Aside
• Codebase grounding: Local clones of Quarto main and reveal.js, the issue reproductions, repository contribution and test documentation, and the existing backport workflow.
• Human review: I reviewed the changes and test results and approved this submission.

[Copilot]

Copilot was unable to review this pull request because the user who requested the review has reached their quota limit.

[posit-snyk-bot]

✅ Snyk checks have passed. No issues have been found so far.

Status	Scan Engine	Critical	High	Medium	Low	Total (0)
✅	Open Source Security	0	0	0	0	0 issues
✅	Licenses	0	0	0	0	0 issues

💻 Catch issues earlier using the plugins for VS Code, JetBrains IDEs, Visual Studio, and Eclipse.