feat(a11y): add ARIA attributes, live regions, and automated accessibility testing

[pls78]

Context

Improves accessibility for screen reader users across analysis components, sidebar collapsibles, bulk editor actions, and notifications.

Summary

This PR can be summarized in the following changelog entries:

• Adds aria-expanded state to sidebar collapsible buttons, aria-label to the notifications region, and replaces anchor-based buttons with semantic <button> elements in the bulk editor.
• Adds live region announcements (aria-live, @wordpress/a11y speak()) so screen readers announce analysis score updates dynamically.
• Adds automated accessibility testing with jest-axe for SidebarCollapsible, AnalysisChecklist, and ContentAnalysis components.

Relevant technical choices:

• @wordpress/a11y speak() is used in Results.js instead of a custom live region, to align with the WordPress accessibility API.
• jest-axe was added as a root devDependency since it is used across multiple packages.

Test instructions

Test instructions for the acceptance test before the PR gets merged

This PR can be acceptance tested by following these steps:

1. Open any post in the block editor with Yoast SEO active.
2. Open the Yoast SEO sidebar panel.
3. Inspect the sidebar collapsible buttons — they should have aria-expanded="true"/"false".
4. Run a screen reader (e.g. VoiceOver): changing post content should trigger spoken announcements of analysis results (e.g. "2 problems, 1 improvements, 3 good results").
5. Navigate to SEO > Bulk Editor — "Save" and "Save all" should render as <button> elements, not <a> links.
6. Check the notifications aside has aria-label="Notifications".
7. Run yarn test — all new and existing tests should pass.

Relevant test scenarios

• Changes should be tested with the browser console open (check for React warnings)
• Changes should be tested on different posts/pages/taxonomies/custom post types/custom taxonomies
• Changes should be tested on different editors (Default Block/Gutenberg/Classic/Elementor/other)
• Changes should be tested on different browsers (screen reader behavior varies)
• Changes should be tested on multisite

Test instructions for QA when the code is in the RC

• QA should use the same steps as above.

Impact check

This PR affects the following parts of the plugin, which may require extra testing:

• Analysis report components (ContentAnalysis, Results)
• Sidebar collapsible component
• Bulk editor list table
• UI library notifications component

Other environments

• This PR also affects Shopify.
• This PR also affects Yoast SEO for Google Docs.

Documentation

• I have written documentation for this change. For example, comments in the Relevant technical choices, comments in the code, documentation on Confluence / shared Google Drive / Yoast developer portal, or other.

Quality assurance

• I have tested this code to the best of my abilities.
• During testing, I had activated all plugins that Yoast SEO provides integrations for.
• I have added unit tests to verify the code works as intended.
• If any part of the code is behind a feature flag, my test instructions also cover cases where the feature flag is switched off.
• I have written this PR in accordance with my team's definition of done.
• I have checked that the base branch is correctly set.
• I have run grunt build:images and commited the results, if my PR introduces new images or SVGs.

Innovation

• No innovation project is applicable for this PR.
• This PR falls under an innovation project. I have attached the innovation label.
• I have added my hours to the WBSO document.

[github-actions]

A merge conflict has been detected for the proposed code changes in this PR. Please resolve the conflict by either rebasing the PR or merging in changes from the base branch.