test: fix flaky test_large_batch_operations

[vdusek]

Summary

• test_large_batch_operations was flaky because fetch_next_request() can temporarily return None due to eventual consistency in the Apify API, even when is_empty() reports requests still exist
• Replaced the while not is_empty + hard assert request is not None pattern with a retry loop that tolerates up to 10 consecutive empty fetches (with 0.5s sleep between retries) before concluding the queue is drained
• The final assertions (processed_count == 500 and is_finished) remain unchanged and still validate correctness

Test plan

• Lint passes (uv run poe lint)
• Unit tests pass (uv run poe unit-tests)
• Integration tests pass in CI (the previously flaky test should now be stable)

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 67.95%. Comparing base (cf36cc2) to head (8cb4a8a).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #780      +/-   ##
==========================================
- Coverage   68.02%   67.95%   -0.08%     
==========================================
  Files          46       46              
  Lines        2696     2696              
==========================================
- Hits         1834     1832       -2     
- Misses        862      864       +2     

Flag	Coverage Δ
unit	67.95% <ø> (-0.08%)	⬇️

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