test: add rerun backoff to test_execution_summary

[asmacdo]

Summary

• Add flaky rerun backoff to test_execution_summary to handle slow CI runners (Mac, PyPy) where the short sleep time doesn't allow enough samples to be collected

Closes #384

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.81%. Comparing base (3756590) to head (6e2c894).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #394   +/-   ##
=======================================
  Coverage   91.81%   91.81%           
=======================================
  Files          15       15           
  Lines        1112     1112           
  Branches      138      138           
=======================================
  Hits         1021     1021           
  Misses         69       69           
  Partials       22       22           

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[Copilot]

Pull request overview

This PR adds retry backoff logic to the test_execution_summary test to handle flaky failures on slow CI runners (Mac, PyPy). The test was failing because insufficient samples were being collected before the monitored process exited, resulting in None values for execution summary metrics.

Changes:

• Added @pytest.mark.flaky(reruns=5) decorator to enable automatic retries
• Implemented backoff mechanism that scales sleep time and report interval based on retry attempt number
• Added pytest request fixture to access retry attempt information

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