Add unit tests for markdownUtils hiddenElements

I noticed while exploring the codebase that markdownUtils hiddenElements doesn't have dedicated unit tests yet. I'd love to help improve the test coverage for this utility!

Why this matters:

1. Reliability: Adding tests will help ensure this utility works as expected now and in the future
2. Confidence: Future contributors can modify related code with more confidence
3. Documentation: Tests serve as living examples of how the utility should behave
4. Quality: Small improvements in test coverage add up to a more robust codebase

What I've done so far:

I've already prepared a PR (#1994) with a set of unit tests for this functionality. The changes are test-only and don't affect any production behavior. The PR is ready for reviews.

I'd appreciate:

Feedback on the test approach in PR #1994
Guidance on any edge cases I might have missed
Your review when you have a moment
This is my first contribution here — I'm excited to help and learn from all of you!
Thank you for maintaining such a wonderful project.

Start by locating the markdownUtils hiddenElements utility and review PR #1994, which contains the proposed unit tests. Compare the tests with the utility's existing behavior and identify any missing edge cases; the work is done when the test suite covers hiddenElements and passes.