test: Resolve flaky webhook integration test

[vdusek]

Summary

• The _get_finished_run_id helper in webhook tests asserted that at least one SUCCEEDED run of apify/hello-world already exists, but this is not guaranteed (runs can be cleaned up between test runs)
• Instead of failing with an assertion error, the helper now falls back to starting a new hello-world actor run via .call() (which waits for completion) when no completed runs are found
• Fixes the flaky test_webhook_test[async] failure seen in CI: https://github.com/apify/apify-client-python/actions/runs/22345359592/job/64658461897?pr=639

Test plan

• Pre-commit hooks pass (lint, type check, docstrings)
• Integration tests pass in CI without the flaky failure

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.62%. Comparing base (7895a4e) to head (9791a2c).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff             @@
##           master     #640       +/-   ##
===========================================
+ Coverage   75.60%   96.62%   +21.02%     
===========================================
  Files          45       45               
  Lines        4271     4271               
===========================================
+ Hits         3229     4127      +898     
+ Misses       1042      144      -898     

Flag	Coverage Δ
integration	94.63% <ø> (?)
unit	75.60% <ø> (ø)

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