PR #30136 review item O6: test_container_restart.py used fixed
`time.sleep(8)` calls after `docker restart` to wait for the
cont-init reconciler to finish. Fixed sleeps are slow when the
event happens fast and false-fail when the event happens slow.
Replace with two polling helpers:
* `_wait_for_path(container, path, kind='f' | 'd', deadline_s=...)`
— generic `test -f/-d` poller. Returns True on success, False on
timeout; callers assert with a clear message.
* `_wait_for_reconcile_log_mention(container, profile, ...)` — the
reconciler's per-profile log line is the canonical signal that
the cont-init reconcile has finished for that profile. Poll on
it instead of a sleep that hopes 8 seconds is enough.
The fixture-level setup wait is similarly migrated: it now polls
for `profile=default` in the boot log (every container always
gets a default-slot entry per item I1) and raises a clear timeout
error from the fixture if the container never finishes cont-init —
much better diagnostics than a mid-test KeyError.
The remaining `time.sleep()` calls are all internal interval_s
between probe attempts; no fixed wait points left.
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