"Other…" option does not respect limit

In checkbox questions, `limit: N` allows participants to select up to N items. However, if `allowOther` is `true`, this allows participants to bypass the limit:

| Unchecked | Checked |
|--------|--------|
| | |

Start by locating the checkbox-question UI and validation paths that handle `limit` and `allowOther`. Reproduce the shown case with a checkbox question using both options, then verify that selecting “Other…” counts toward the configured limit without allowing extra selections.

Written by the indexing model from the issue text.