[A11Y-196] Restore the V0 link text label association

[dancormier]

https://stackoverflow.atlassian.net/browse/A11Y-196

Summary

Restore the link-text label fix from #530 to the V0 release line. It shipped in 0.15.5 but was missing from the branch used to release 0.16.0, causing an accessibility regression. This patch retains WebP support and prepares a 0.16.1 release.

Changes

• Match the Link text input ID to its visible label’s for attribute, restoring the native label association required by WCAG 1.3.1.
• Backport regression coverage for both link-dialog labels and verify their associated controls.
• Add browser coverage for accessible field names, keyboard focus, and saved link text/destination.
• Add a patch changeset; no dependency or styling changes.

This is a focused backport of the SO_49 label fix, not a reimplementation of the broader A11Y-196 toolbar work.

How to test

Using Node 24 LTS:

1. Run npm ci, then npm start -- --port 8080.
2. Open http://localhost:8080/, select rich-text mode, and open the Link dialog.
3. With a screen reader enabled, confirm the fields are announced as Link URL and Link text. Tab from the URL field to Link text; focus should remain visible.
4. Enter a URL and link text, then save. Confirm the destination and displayed text are preserved.
5. Run npm run test:unit -- --runInBand, npm run test:e2e, npm run test:release-config, npm run lint, npx tsc --noEmit, and npm run prepublishOnly.

Locally, 1,094 unit tests, all 198 browser tests across Chromium/Firefox/WebKit, and 8 release-config tests passed. Lint, type-check, and packaging build passed with existing warnings. Browser tests used an isolated server on port 8082.

The new unit test reproduced the failure before the fix. Compiled source/type comparison against published 0.15.5 showed only the intended WebP default difference after restoration.

Manual screen-reader checks and verification in consuming applications remain pending.

Preview: Open deploy preview.

Success criteria

From A11Y-196; unchecked items remain outside this focused backport or require downstream verification:

• The Link text input has a programmatic label matching its visible label.
• Toolbar actions expose valid button semantics and useful accessible names.
• Every toolbar action is operable without a mouse.
• Toolbar focus order is logical and focus remains visible.
• Tooltip/help content needed to understand a control is available on keyboard focus.
• The fix is verified in question, article, and tag-history editors.
• Existing editor shortcuts and formatting behavior continue to work.

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