* fix(background): treat POSIX zombie leader as already-exited
terminateOwnedProcess always returned leaderAlreadyExited=false, so
TerminateCommand surfaced "did not exit after SIGKILL" when ps was
unavailable and the leader was already a waitable zombie. Probe the
leader the way Windows uses GetExitCodeProcess, then still signal the
group so live descendants are not skipped.
* fix(background): preserve live-group termination failures
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Co-authored-by: cairn-intern <[email protected]>
