Add Comment To Workflow Run Turn

Request body for leaving a comment. Everything else on CommentConfig is server-assigned.

Separate from CommentConfig on purpose. That model is also the stored shape and is used to read records written before any of these rules existed, so constraining it would make legacy documents — including comments stored with null content — unreadable, and would fail the run fetch for every run holding one. Rules belong at the boundary, where new input arrives.

Mirrors the RatingConfig / RatingRequest split in configs/rating.py.

Response model for the turn comment endpoints. Scoped to the mutated turn for the same reason as RatingsResponse.