Expand validation test coverage at atelet's RPC layer

Follow-up from review discussion on #206 (https://github.com/agent-substrate/substrate/pull/206#discussion_r3385245614).

#206 (in review) validates path-construction inputs at atelet's RPC boundary. The negative path is tested through the public surface (`TestRPCBoundariesReject` calls the real `Run`/`Checkpoint`/`Restore` handlers and asserts `InvalidArgument`), but the positive path cannot be asserted through the RPCs: after validation passes, the handlers immediately need object storage, the actor dirs under `/var/lib/ateom-gvisor` (`ateompath.BasePath` is a const), image pulls, and a live ateom.

Making the happy path testable means making those dependencies fakeable (injectable storage and dialer, a redirectable base path). That refactor is the bulk of this issue; the additional tests follow from it. It would also be a good foundation for unit testing the checkpoint/restore flows generally, which today are only covered by e2e.