test: replace fixed sleep with polling for RQ stats assertions

[vdusek]

Summary

• Added poll_until_condition() helper to tests/integration/_utils.py — polls an async callable until a condition is met or a 60-second timeout expires (5-second intervals).
• Replaced await asyncio.sleep(10) + single-fetch patterns in 7 request queue integration tests with poll_until_condition, fixing intermittent failures where 10 s was not enough for Apify's eventually-consistent stats API to propagate.

The following tests were intermittently failing in CI due to this timing issue:

• test_request_queue_metadata_another_client[single]
• test_crawler_run_request_queue_variant_stats[Full rq client]
• test_request_queue_has_stats[shared]
• test_request_queue_deduplication_unprocessed_requests[shared]
• test_request_queue_deduplication_use_extended_unique_key[single]

Failed CI runs that triggered this fix:

• https://github.com/apify/apify-sdk-python/actions/runs/24509008169/job/71634833798 (Python 3.10 — 4 failures)
• https://github.com/apify/apify-sdk-python/actions/runs/24509008169/job/71634833797 (Python 3.14 — 1 failure)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.57%. Comparing base (390ccaf) to head (049a6d4).
⚠️ Report is 3 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #864   +/-   ##
=======================================
  Coverage   86.57%   86.57%           
=======================================
  Files          48       48           
  Lines        2920     2920           
=======================================
  Hits         2528     2528           
  Misses        392      392           

Flag	Coverage Δ
e2e	37.87% <ø> (ø)
integration	59.28% <ø> (ø)
unit	74.62% <ø> (ø)

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