On a launchd-managed gateway (macOS), /restart stopped the gateway but
never relaunched it: the handler's service detection checks only
INVOCATION_ID (systemd) and container markers, so under launchd it takes
the detached path and exits 0 — which KeepAlive.SuccessfulExit=false
treats as a deliberate stop. The gateway stays silently dead until a
manual launchctl kickstart.
Detect launchd via XPC_SERVICE_NAME, which launchd sets to the job label
for processes it spawns. The probe deliberately excludes the literal
"0": interactive macOS shells inherit XPC_SERVICE_NAME=0 (a truthy
string), and routing an unsupervised interactive gateway to the service
path would make it exit non-zero with nothing to revive it.
Routing through via_service=True (rather than forcing a non-zero exit
on the detached path) matters: the detached path also spawns a helper
that relaunches the gateway, so exiting non-zero there would have BOTH
the helper and launchd respawn it — two gateways racing for the same
bot tokens. The service path spawns no helper; launchd is the single
respawner.
Fixes#43475. Supersedes the run.py-era probes in #19940/#33393 (the
handler has since moved to gateway/slash_commands.py) and avoids the
double-spawn risk in the exit-code-site approaches (#43498, #43596).
