fix(session-host): skip identity checks for a zombie group leader

[Quorafind]

What

Fix a macOS-only failure in the background-job stop path. On macOS, Stop raised
could not verify the identity of process N after it had already terminated its own
worker, so the job could not be reported as stopped.

Root cause

process_is_running does not agree across platforms about a zombie:

• process_liveness.rs:550 (Linux) reads /proc/<pid>/stat and reports a zombie as
  not running.
• process_liveness.rs:571 (every other Unix, macOS included) asks kill(pid, 0) and
  reports a zombie as running.

wait_for_recorded_unix_group_exit guarded its identity check with
process_is_running(pid) == Some(true), so on macOS it asked proc_pidinfo for the
identity of a zombie — which answers nothing — and verify_group_leader_identity
failed the whole stop. On Linux the same guard skips the check, which is why this never
showed up there. The zombie in question is the leader the stop itself had just killed,
still unreaped by its parent.

Change

One named rule (verify_live_group_leader_identity) shared by both call sites: the
identity is verified while the leader is still an active group member (the member
enumeration already filters zombies), and skipped once it is a zombie. A zombie's pid
cannot be reused until it is reaped, so its identity cannot have been taken over.

The checks before signalling are untouched — terminate_recorded_unix_group still
verifies the leader twice before it signals the group, so the fail-closed property for a
live leader is unchanged.

Evidence

Seven tests fail on macos-14 without this change, all in the test job of a tag run:

process_liveness::tests::mismatched_live_leader_identity_is_never_group_signalled
stop_job::tests::local_takeover_terminates_and_verifies_a_distinct_owner
stop_job::tests::stop_retries_termination_when_stopped_job_still_records_a_live_worker
stop_job::tests::stop_terminal_job_terminates_lingering_exact_owner_without_relabelling
stop_job::tests::stopping_a_succeeded_job_ends_its_still_resident_worker
stop_job::tests::stop_best_effort_stops_live_owner_without_recorded_identity
stop_job::tests::local_takeover_best_effort_stops_live_owner_without_recorded_identity

with process_liveness.rs:997 panicking on could not verify the identity of process 20402, and the stop_job tests reporting worker exit was not verified.

Verification

• cargo check -p rebon-session-host --locked (Windows host): passes.
• rustfmt --check on the changed file: clean.
• cargo check --all-targets --target x86_64-apple-darwin and
  --target x86_64-unknown-linux-gnu cannot run on this host: cc-rs has no cc /
  cross gcc, and the workspace pulls C dependencies. The macOS behaviour this fixes
  therefore has to be confirmed by this PR's test job (macos-14), which is why it is a
  PR rather than a direct push.