Edit one comment on a question of an answer request

Rewrites the text of a single comment in the thread on one question of one answer request — the write behind the inline edit in the comment popover on the answer and review pages. Only the comment author may edit it; anyone else gets 403 cannot_update_question_comment, including other people at the same company. The comment must be filed against the addressed question and request, otherwise 404 question_comment_not_found — the same code an unknown comment answers, so the endpoint never confirms a comment the caller cannot address exists elsewhere. The edit rewrites the updatedBy* fields, which is what both frontends render as the comment author; createdBy* is immutable.