Read one evaluation dataset

A dataset as the public API describes it.

Every field is REQUIRED and nullable rather than optional: the route always emits all of them, and a default here would say the key may be absent, which is a different contract from "present and null". The shape roster compares this to the Zod side field-for-field, so the two cannot drift apart on that distinction.

dataset_id is the id a CLIENT addresses the dataset by — its own client_dataset_id when it created the dataset, or the row id for one authored in the UI. key is the pre-image of that id, so a pulled dataset recovers its key when key and name differ.

No case count: no dataset read computes one, and deriving it would need an N+1 over versions. It lives on a version, where it is one grouped aggregate.