Date range filtering for the cache runs API

The cache runs listing endpoint now supports a ran_at date range filter, so reporting clients can request runs within a specific execution window without paginating the full history. The filter accepts Unix timestamps in seconds using field-scoped operators: ran_at[gt], ran_at[gte], ran_at[lt], and ran_at[lte]. Invalid or out-of-range timestamps now return a bad request with a concrete validation message instead of silently being ignored. Cache run responses also serialize ran_at from the stored execution time rather than from created_at, and the generated Swift server contract and CLI client files now expose the typed ran_at query payload and surface validation failures.