Filter cache runs by execution date range

The cache runs listing endpoint now accepts date range filters via a ran_at query object, accepting ran_at[gt], ran_at[gte], ran_at[lt], and ran_at[lte] with Unix timestamps in seconds. This lets reporting clients request cache runs within a specific execution window instead of paginating the full history. Invalid or out-of-range timestamp filters return a bad request with a concrete message. Cache run responses also serialize ran_at from the stored execution timestamp rather than deriving it from created_at. Generated server contract and Swift command-line client files are updated to expose the new filter and validation handling.