fix(utils): strip MDX export blocks before heading detection in parseMarkdownContentTitle

[okxint]

parseMarkdownContentTitle strips leading MDX import blocks via a regex before looking for # h1, but does not strip export declarations (e.g. export function Author() {...} or export const meta = ...). When an MDX file has an export before the heading, the stripped content still starts with export ..., the title regex finds no match at position 0, and Docusaurus falls back to the site name for <title> and the sidebar.

Fix: add export as an alternative alongside import in the existing regex. The change is minimal — import\s becomes (?:import|export)\s — and the existing import-stripping behaviour is preserved exactly.

Fixes #12331

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[SethFalco]

(Note, I'm not a maintainer, just the author of the original issue. I'm weighting in with my review but when another maintainer chimes in, their feedback should get precedence.)

The fix itself looks acceptable to me.

In the JSDoc it explicitly stated before that the function "[ignores] import/export declarations", which wasn't true before. This PR is what makes the documentation accurate!

I'd expect to see at least one new test cases added too in packages/docusaurus-utils/src/__tests__/markdownUtils.test.ts:

  it('parses markdown h1 title placed after export declarations and remove it', () => {
    const markdown = dedent`
          export function Uwu() {
            return <>uwu</>;
          }

          # Markdown Title

          <Uwu/> Ipsum

        `;

    expect(
      parseMarkdownContentTitle(markdown, {removeContentTitle: true}),
    ).toEqual({
      content: markdown.replace('# Markdown Title\n', ''),
      contentTitle: 'Markdown Title',
    });
  });

While reviewing locally, I had added this test case as well, but I see it would fail as the title parsing won't actually render any JSX that may appear in headings. The title is set to <Uwu/> Title literally. As the doc says, the function "won't be best effort", so in this PR at least, I don't think any changes to fix this would be worthwhile. Just making note of it and leaving the test case for future reference. I personally don't have a use case to put JSX in headings anyway, so happy to pretend I didn't see that.

  it('parses markdown h1 title placed after multiple export declarations used in h1 and and remove it', () => {
    const markdown = dedent`
          export function Uwu() {
            return <>uwu</>;
          };

          # <Uwu/> Title

          Lorem Ipsum

        `;

    expect(
      parseMarkdownContentTitle(markdown, {removeContentTitle: true}),
    ).toEqual({
      content: markdown.replace('# <Uwu/> Title\n', ''),
      contentTitle: 'uwu Title',
    });
  });