The group-leadership guard nulls a non-leader pgid so the shared group is
never killpg'd, but that also skipped the helpers-outlive-leader sweep
(#76759): a SIGHUP-ignoring helper of a shared-group child kept the PTY
slave open past close(). Snapshot the child's descendants with psutil
BEFORE the parent is signalled (they reparent and become undiscoverable
once it exits), then send SIGHUP with the helper grace and SIGKILL the
survivors individually — mirroring browser_tool_lifecycle's non-leader
branch and the group sweep's leader-dead cadence.
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