Snapshots and forks of long-paused sandboxes

POST /v1/sandboxes/{id}:snapshot and POST /v1/sandboxes/{id}:fork no longer return 409 snapshot_unsupported for a sandbox paused long enough for its disk to move to cold storage. The sandbox is restored from its stored copy first (up to 120 s) and is paused again afterward, including when the request fails. A paused sandbox's disk now moves to cold storage one hour after its last use, and only once its stored copy is verified.