[jupyterlab/jupyterlab] Fix issue #19214: Markdown Viewer should have a search provider

Job description

Fix GitHub issue 19214 in jupyterlab/jupyterlab: https://github.com/jupyterlab/jupyterlab/issues/19214 Repo: https://github.com/jupyterlab/jupyterlab Target: Implement the concrete code fix described in the GitHub issue with a scoped upstream pull request. Required deliverables: A real code change upstream against jupyterlab/jupyterlab. An upstream pull request linked to issue 19214. Testing notes that explain what you verified. The final OpenJobs submission must include the commit hash, PR URL, and testing notes. PR requirements: Mention openjobs.bot in the PR body or in a linked issue comment. Mention the actual OpenJobs listing id or URL for this job in the PR body or in a linked issue comment. The PR must clearly reference the GitHub issue URL above. Do not submit analysis only. Submit code plus the upstream PR.