fix(server): keep Error-phase sandbox records through the prune sweep

[pkhodade-NV]

Summary

• The periodic store-vs-backend reconciliation sweep (reconcile_store_with_backend / prune_missing_sandbox in openshell-server) deletes any persisted sandbox record not present in the driver's live backend snapshot, except for Completed and failed-main-process phases.
• A compute driver whose registry is purely in-process and never rehydrates after a restart (no persistence of its own -- e.g. MXC) reports every previously-known sandbox as "missing" on the very first sweep after gateway startup, including ones already correctly, terminally marked Error by earlier crash detection.
• Result: the sweep silently deletes those Error-phase records shortly after a gateway restart, racing any client (GetSandbox/ListSandboxes/DeleteSandbox) working with the same sandbox in that window -- some requests see the full record, others see it as already gone, depending purely on timing against the background sweep.

Related Issue

No linked issue -- this is a localized correctness fix to a background reconciliation sweep's phase handling, disproving and superseding an initial hypothesis (that GetSandbox and ListSandboxes read from different backing stores) with a confirmed root cause after tracing both RPC handlers and the sweep itself.

Changes

• compute/mod.rs: prune_missing_sandbox now treats SandboxPhase::Error the same as the existing Completed exemption -- both are already-settled, informational terminal states with no live compute resource left to reclaim, so the durable record is kept instead of deleted. (Stopping/Stopped/Starting sandboxes are unaffected -- they're still re-marked Error with a ComputeResourceMissing condition, as before.)

Testing

• Added prune_missing_sandbox_keeps_error_phase_records, mirroring the existing prune_missing_sandbox_releases_driver_resources test harness: puts an Error-phase sandbox in the store, runs a sweep with zero grace period, asserts the record survives with its phase unchanged and no driver delete call was made.
• Full compute:: module test suite passes: cargo test -p openshell-server --target x86_64-pc-windows-msvc --lib compute::.

Checklist

• Tests added for the new behavior
• No unrelated changes bundled in

Originally opened as GitLab MR !116 against our internal mirror; re-opened here against windows for upstream review.

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