[snapshot] restored boxes lose persisted commands without OCI metadata

[ZhiXiao-Lin]

Summary

A box restored from a filesystem snapshot persists the original cmd in boxes.json, but start ignores it when the restored layout has no OCI image config. Guest-init receives the fallback echo No command specified; exec /bin/sh, so the restored workload exits immediately with code 0.

Reproduction

1. Run a box with an explicit command that writes a marker and stays alive.
2. a3s-box snapshot create <box>.
3. a3s-box snapshot restore <snapshot> --name restored.
4. Confirm a3s-box inspect restored contains the original command.
5. a3s-box start restored.

Observed on Windows/WHPX: the restored record becomes dead/exit 0 and init-rust.log shows BOX_EXEC_ARG_1=echo No command specified; exec /bin/sh.

Root cause

boot_from_record correctly reconstructs BoxConfig.cmd. However, VmManager::build_instance_spec only applies config.cmd inside the Some(oci_config) branch. Snapshot-restored layouts can have layout.oci_config == None; that branch always installs the no-command fallback, even when the persisted command/entrypoint override is non-empty.

Expected behavior

Persisted command and entrypoint overrides must be authoritative even when OCI metadata is unavailable. The fallback shell should only be used when neither an OCI command nor an explicit persisted command/entrypoint exists.

Validation needed

• Unit regression for a guest-init layout with no OCI config and a non-empty command override.
• End-to-end snapshot create/restore/start test that proves the restored filesystem marker is present and the restored command remains running.

[ZhiXiao-Lin]

Fix and validation are pushed in 22947a1e13d36a8d2e71ea816a77026de65af23e on fix/windows-whpx-validation-20260720.

build_instance_spec now preserves the persisted command/entrypoint when a restored layout has no OCI config; the fallback shell is used only when no explicit persisted command exists.

Validation:

• runtime regression test_resolve_entrypoint_without_oci_preserves_overrides passed
• real WHPX snapshot flow passed end to end: create, inspect, restore, start, restored marker in logs, export, and restored file verification
• committed-image command override and re-run passed in the same workflow
• runtime suite: 880 passed / 1 ignored

Keeping this issue open until the branch is merged.

[ZhiXiao-Lin]

Current main-targeting integration is PR #134, now at a6ed30886eb6ae8d2050a3ea0c20f548d40f9b27.

I reran the end-to-end case on real WHPX with current artifacts: create/start the source command, observe its filesystem marker, stop, snapshot, restore under a new name, confirm the restored layout has no OCI config, and start it. The restored record retained the original command, logged SNAPSHOT_RESTORED_MARKER_OK, and remained running until explicitly stopped.

Keeping this issue open until #134 is merged.