Checking a box in an issue description consistently throws an error

What does this MR do?

CommonMark can create "tight lists" and "loose lists" (which have <li> content wrapped in <p> tags). We were using too strong a selector when looking for the checkbox.

Changed this

html.css(".task-list-item[data-sourcepos^='#{line_number}:'] > input.task-list-item-checkbox").first

to

html.css(".task-list-item[data-sourcepos^='#{line_number}:'] input.task-list-item-checkbox").first

What are the relevant issue numbers?

Closes #57604 (closed)

Does this MR meet the acceptance criteria?

• [ ] Changelog entry added, if necessary
• [ ] Documentation created/updated via this MR
• [ ] Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• [ ] Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• [ ] Conforms to the database guides
• ~~[ ] Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.~~
• Security reports checked/validated by reviewer