Validate inline code highlight metadata in CI

[aurorascharff]

Summary

Invalid inline code highlight metadata currently throws from CodeBlock at runtime, which can leave an otherwise valid documentation page showing a client-side application error. This happened after #8520 moved <form action={submitAction}> from line 23 to line 25 without updating its highlight metadata. The broken Server Functions page was reported in #8523 and fixed separately in #8531.

This PR extends the existing Markdown ESLint rule to validate inline code highlights before deployment. It checks that:

• the metadata is valid JSON with the expected tuple structure;
• the referenced line exists in the code block;
• the highlighted text exists on that line; and
• repeated text specifies a fromIndex.

The validator uses the normalized metadata produced by the Markdown parser, matching what MDX passes to CodeBlock and supporting highlights that contain escaped quotes. Regression fixtures cover valid metadata, missing text, and out-of-bounds lines. The local-rule tests now run as part of ci-check.

Testing

• yarn test:eslint-local-rules
• yarn eslint "src/content/**/*.md"
• yarn ci-check

To verify the regression, I temporarily changed the Server Functions example's valid submitAction highlight from line 25 to line 24. ci-check failed with:

server-functions.md:115 — Could not find 'submitAction' on highlighted line 24

After restoring line 25, the complete ci-check passed.

[github-actions]

Size changes

Details

📦 Next.js Bundle Analysis for react-dev

This analysis was generated by the Next.js Bundle Analysis action. 🤖

This PR introduced no changes to the JavaScript bundle! 🙌

[eps1lon]

Immediate PTSD when thinking about how much time I spent getting these highlight tupples right.