* fix(sandbox): leave SIGTERM/SIGINT teardown to the Gateway lifespan
AIO and E2B providers built on the event-loop thread installed SIGTERM/SIGINT
handlers that force-destroyed every sandbox synchronously, before uvicorn
started the run drain. The Gateway lifespan now holds sandbox shutdown
ownership from its first step, so providers skip those handlers and the
existing hook closes the provider after run drain and MCP teardown. SIGHUP
keeps the provider fallback. AIO acquire now refuses once shutdown began.
* docs(sandbox): document Gateway-owned sandbox shutdown and AIO admission fence
* test(gateway): inspect the lifespan body now that lifespan wraps it
* docs(changelog): reference #7560 for the sandbox SIGTERM drain-order fix
* ci: retrigger sandbox image smoke after a registry pull timeout