Fix coverage handling for stuck scenario warning

[lola831]

Description

This fixes a coverage-only failure in test_scenario_stuck.

The new stuck-scenario test passes in a normal test run, but fails under the coverage workflow because src/scenic/core/dynamics/scenarios.py suppresses the warning whenever sys.gettrace() is set. Under pytest --cov, tracing is enabled, so the warning is skipped and the test fails.

behaviors.py already has logic to allow the warning under coverage while still suppressing it for a real debugger. This PR makes scenarios.py consistent with that behavior.

Issue Link

N/A

Checklist

• I have tested the changes locally via pytest and/or other means
• I have added or updated relevant documentation
• I have autoformatted the code with black and isort
• I have added test cases (if applicable)

Additional Notes

N/A

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.69%. Comparing base (07625cb) to head (15342f6).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #444      +/-   ##
==========================================
+ Coverage   89.66%   89.69%   +0.03%     
==========================================
  Files          48       48              
  Lines       13193    13194       +1     
==========================================
+ Hits        11829    11834       +5     
+ Misses       1364     1360       -4     

Files with missing lines	Coverage Δ
src/scenic/core/dynamics/scenarios.py	94.59% <100.00%> (+1.09%)	⬆️

... and 5 files with indirect coverage changes

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[dfremont]

Thanks, Lola!