Create several eval cases

Adds up to 100 cases to the suite in one call — the bulk form of the single create, and the way to import a suite or convert a repo's test files without a round trip per case. Every entry needs a title and a non-empty steps array. Cases are validated together and reported individually: a failed entry does not roll back its committed siblings, so the response is 201 even when some entries were refused — read failed rather than branching on the status alone. Send an Idempotency-Key header to make a retry land on the same rows.