A sandbox that stops responding is reported within 5 seconds, and a restart no longer reports a kept disk as deleted

A running sandbox that stops responding now returns unreachable, with since and reason (agent_not_answering, disk_stalled or host_quarantined), from GET /v1/sandboxes/{id}, and the new events sandbox.unreachable and sandbox.reachable are sent to webhooks. Previously such a sandbox returned running with no indication. When the sandbox's server cannot finish writing to its disk, a new command now returns sandbox_unreachable (503, retryable) after 5 seconds, with details.reason set to disk_stalled. Previously the request waited about 30 seconds and returned guest_busy (429). The request runs nothing, and commands already running continue. POST /v1/sandboxes/{id}:restart on a running sandbox whose server has not confirmed the reboot's stop now returns sandbox_unreachable (503, retryable) and keeps the disk. Previously it returned not_resumable (409) with diskKept: false. A retry with the same Idempotency-Key waits for the stop, then starts the sandbox from its disk. A command sent to a stopping sandbox now returns sandbox_stopped with "is stopping" instead of "is stopped (ended earlier)". When the server could not pause a sandbox to free disk space, the sandbox keeps running instead of refusing every call. See troubleshooting.