When the gateway receives SIGUSR1 (graceful restart via launchd_restart),
the SIGUSR1 handler calls request_restart(via_service=True) and the
gateway shuts down cleanly with exit code 0.
However, the generated launchd plist uses KeepAlive → SuccessfulExit →
false, meaning launchd only relaunches on *non-zero* exit codes. A
clean exit(0) is treated as "successful, don't restart", so the
gateway stays down after /restart, /update, or SIGUSR1.
The systemd unit template already uses RestartForceExitStatus=75 for the
same scenario. Mirror that convention: when _restart_via_service is
True, raise SystemExit(75) so launchd's SuccessfulExit=false policy
triggers a relaunch.
Closes#28135
5987b24314