test: correct isinstance check in webhook integration test

[vdusek]

Summary

• Fix flaky test_webhook_create_and_get integration test
• The _get_finished_run_id helper incorrectly asserted isinstance(run, ListOfRuns) after calling .call(), which returns a Run object — causing failures when no prior successful runs existed for the hello-world actor

Test plan

• Verify the fix by inspecting the corrected assertion (isinstance(run, Run))
• Re-run integration tests to confirm the flaky test passes consistently

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.59%. Comparing base (21f2665) to head (2550044).
⚠️ Report is 3 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #662      +/-   ##
==========================================
+ Coverage   96.45%   96.59%   +0.13%     
==========================================
  Files          45       45              
  Lines        4318     4318              
==========================================
+ Hits         4165     4171       +6     
+ Misses        153      147       -6     

Flag	Coverage Δ
integration	94.74% <ø> (+0.27%)	⬆️
unit	78.04% <ø> (ø)

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