fix(core): recognize arbitrary HTML anchors in broken anchor checker (AI-assisted)

[Sanjays2402]

Fixes #9808

Problem

The onBrokenAnchors checker only knew about anchors explicitly collected through the useBrokenLinks() API (e.g. by the @theme/Heading component). Valid anchors written directly in MDX/React pages — such as <div id="anchor"/>, <h3 id="anchor">, or legacy <a name="anchor"/> — were never collected, so links targeting them were wrongly reported as broken anchors during docusaurus build, even though the anchors work fine in the browser.

Fix

During SSG (packages/docusaurus/src/client/serverEntry.tsx), anchor targets are now extracted from the fully rendered HTML page — the same source of truth browsers use for fragment navigation — and added to the collected anchors:

• new extractHtmlAnchors() util (packages/docusaurus/src/client/extractHtmlAnchors.ts) collects the id attribute of any element plus legacy <a name="..."> anchors, with HTML-entity decoding, deduplication, and guards against false positives (data-id, text content, non-<a> name attributes)
• docs updated (website/docs/api/docusaurus.config.js.mdx): onBrokenAnchors no longer claims anchors must be "declared with the Heading component"

This matches the direction discussed in the issue (automatically supporting <div id="anchor"/>, <a id="anchor"/>, etc. in Markdown files, not just headings).

Test plan

• New unit tests packages/docusaurus/src/client/__tests__/extractHtmlAnchors.test.ts: 9 tests pass (vitest) — arbitrary element ids, legacy <a name>, mixed pages, dedup, empty anchors, false-positive guards (data-id, aria-labelledby, text content), HTML-entity decoding
• tsc --noEmit --strict (with repo flags noUncheckedIndexedAccess, erasableSyntaxOnly): clean
• oxfmt --list-different on changed TS files: clean
• End-to-end logic simulation of the issue scenario: page HTML containing <a name="anchor"> + link #anchor — before the fix the checker's anchor-matching logic reports #anchor as broken (reproduces the issue); after adding the HTML-extracted anchors, #anchor and #custom-anchor resolve while a genuinely missing #missing anchor is still reported as broken
• Full monorepo yarn build / jest suite not run here (deps not installed in this environment); CI will cover it

(AI-assisted)