fix(utils): stop createExcerpt leaking a multi-line JSX element into the description

[gunjanjaswal]

Motivation

Fixes #12214.

createExcerpt builds the auto description / og:description by scanning a doc line by line. It strips HTML/JSX with /<[^>]*>/g, which only matches a tag that opens and closes on the same line. When the first content node is a multi-line JSX/MDX element, only its first line is seen (e.g. <MyComponent), the regex finds no >, nothing is stripped, and that partial tag becomes the page description — e.g. <meta name="description" content="<MyComponent">.

This adds an inHTML state next to the existing inImport / inCode handling: when a line opens a tag with no > on the same line, skip lines until it closes. Single-line and self-closing tags are unchanged (the existing regex still handles them). The guard only triggers when a line starts (after whitespace) with <Tag and has no >, so it doesn't touch prose or current behavior.

Test Plan

Added creates excerpt after a multi-line JSX element in markdownUtils.test.ts. It fails before the change (excerpt is <MyComponent) and passes after. Runs with the existing docusaurus-utils unit tests.

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