Get Example By Id

A predicted example from a arbiter. Used for both ingestion (POST) and retrieval (GET) of examples.

Split contract: "none" is the ClickHouse marker for unannotated rows. Callers MUST NOT send "none" explicitly — omit split and let get_split decide. When ground_truth is set, get_split randomly assigns train/test (80/20); when it isn't, split is forced to "none". Downstream consumers (GEPA, scoring, dataset export) filter on split in ('train','test'), so a row stuck at "none" is invisible to them. The same rule lives on jobs/schemas.py::BatchExample; if you change one, change the other.