Upgrade erusev/parsedown from 1.7.4 to 1.8.0

[mikelittle]

Summary

Apropros this Dependabot PR which was closed, because this is master and there are security fixes, I looked into it.

• Upgrade erusev/parsedown from ~1.7.4 to ~1.8.0, the first stable release in 6+ years, which includes security fixes (catastrophic backtracking prevention) and improved CommonMark compliance
• Update MarkdownParser::blockHeader() to use the 1.8.0 element structure where handler is now an array with function/argument/destination keys, and child elements use the elements key directly
• Fixes a pre-existing bug where header anchor IDs were empty (href="#" id="") because sanitize_title_with_dashes() was processing raw Parsedown inline tokens instead of plain text — 1.8.0's handler['argument'] provides clean text, so anchors now render correctly

Test plan

• Verify documentation pages render correctly in the admin (Welcome, Getting Started, Linking pages)
• Verify header anchor links contain correct IDs and are clickable
• Verify relative links and cross-module links resolve correctly
• Verify images with relative paths display correctly
• Verify code blocks render with syntax highlighting
• Check PHP error logs for any warnings or errors from the parser

🤖 Generated with Claude Code

[mikelittle]

Some before and after images
before:

after:

[github-actions]

Successfully created backport PR for v26-branch:

• [Backport v26-branch] Upgrade erusev/parsedown from 1.7.4 to 1.8.0 #706

[github-actions]

Backport failed for v12-branch, because it was unable to cherry-pick the commit(s).

Please cherry-pick the changes locally and resolve any conflicts.

git fetch origin v12-branch
git worktree add -d .worktree/backport-704-to-v12-branch origin/v12-branch
cd .worktree/backport-704-to-v12-branch
git switch --create backport-704-to-v12-branch
git cherry-pick -x f698bcbdd7ae0c6f0b7fc500657d6ca3144a7e3f

[github-actions]

Successfully created backport PR for v23-branch:

• [Backport v23-branch] Upgrade erusev/parsedown from 1.7.4 to 1.8.0 #707

[github-actions]

Successfully created backport PR for v24-branch:

• [Backport v24-branch] Upgrade erusev/parsedown from 1.7.4 to 1.8.0 #708

[github-actions]

Successfully created backport PR for v25-branch:

• [Backport v25-branch] Upgrade erusev/parsedown from 1.7.4 to 1.8.0 #709

[github-actions]

After resolving conflicts, continue with:

# Add the resolved files
git add .
# Continue the cherry-pick
git cherry-pick --continue
# Push the branch to GitHub
git push --set-upstream origin backport-704-to-v12-branch
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktree/backport-704-to-v12-branch

Then create a pull request using the GitHub CLI:

gh pr create --base v12-branch --head backport-704-to-v12-branch --title "[Backport v12-branch] Upgrade erusev/parsedown from 1.7.4 to 1.8.0" --body "Backport of #704 to \`v12-branch\`."