Fix flaky commit comment task list spec

What

Wait for the task list checkboxes to be enabled before clicking, and wait on the persisted note content instead of wait_for_requests.

Why

Checkboxes render disabled until TaskList JS enables them. Clicking a disabled checkbox raises nothing and silently does not toggle, so the clicks did nothing, wait_for_requests returned with nothing in flight, and the reload found no checked boxes.

Confirmed by forcing the checkboxes disabled: no error, markdown unchanged, 0 checked after reload. The reload-beats-PATCH theory did not reproduce. 30/30 passes, vs 29/30 unfixed.

References

• https://gitlab.com/gitlab-org/quality/test-failure-issues/-/work_items/44253