test: Fix flaky *_collection_iterate tests with polling helper

[vdusek]

Summary

Tests are still flaky due to eventual consistency - newly created storages aren't immediately visible in the listing endpoint. Adding a polling helper.

Tested with 50 runs, no flakiness.

Failing CI run

• https://github.com/apify/apify-client-python/actions/runs/26557636852/job/78232781171

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.74%. Comparing base (1f92625) to head (8d4f115).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #831      +/-   ##
==========================================
+ Coverage   94.72%   94.74%   +0.01%     
==========================================
  Files          48       48              
  Lines        5045     5045              
==========================================
+ Hits         4779     4780       +1     
+ Misses        266      265       -1     

Flag	Coverage Δ
integration	93.12% <ø> (+0.15%)	⬆️
unit	83.52% <ø> (ø)

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[janbuchar]

Cool, LGTM