Delete an owned input Asset and erase its bytes

Erases the bytes of an input asset the caller owns and keeps a tombstone; a later read answers 404 ASSET_NOT_FOUND. A job already admitted from the asset keeps its own snapshot copy and is not affected; that copy is erased when the job is deleted or its input retention lapses. A result asset is deleted only together with its job (409 ASSET_OWNED_BY_JOB). Idempotent: an asset already deleted, purged or expired answers 204. Requires no product scope.