test: Deflake test_schedule_list and test_task_list by polling eventually consistent listings

[vdusek]

test_schedule_list and test_task_list failed in CI (schedule run, task run) 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.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.61%. Comparing base (a8a393b) to head (2ba22cf).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #951   +/-   ##
=======================================
  Coverage   94.61%   94.61%           
=======================================
  Files          58       58           
  Lines        5239     5239           
=======================================
  Hits         4957     4957           
  Misses        282      282           

Flag	Coverage Δ
integration	92.67% <ø> (ø)
unit	83.73% <ø> (ø)

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