test: Deflake test_schedule_list and test_task_list by polling eventually consistent listings (#951)

`test_schedule_list` and `test_task_list` failed in CI ([schedule
run](https://github.com/apify/apify-client-python/actions/runs/29497290807/job/87617167353),
[task
run](https://github.com/apify/apify-client-python/actions/runs/29498108478/job/87619855853))
because they assert read-your-write on listing endpoints: they list
resources immediately after creating them, and under load the listing
can serve a view that hasn't yet caught up with the creates, so the
fresh IDs are sometimes missing. The creates themselves succeeded in
both cases, so these are eventual-consistency flakes, not client bugs.

The fix wraps each list read in the existing `poll_until_condition`
helper (30 s ceiling), waiting until the created IDs appear in the
listing. The original assertions still run on the final page, so a real
regression still fails. Follows the same deflaking pattern as #824,
#831, #844, and #868.