[codex] Fix microsim self-employment smoke test

[daphnehanse11]

Summary

• update the microsimulation smoke test to treat self-employment income as nonzero when its aggregate magnitude is nonzero
• document why a weighted self-employment total can be negative
• add a changelog fragment

Why

The failing GitHub Actions job on PR #7977 was not caused by the Texas Medicaid change. It failed in policyengine_us/tests/microsimulation/test_microsim.py because the test assumed self_employment_income.sum() > 0.

That assumption is too strict: self_employment_income is modeled as net business income, so weighted losses can outweigh weighted profits in a sampled dataset and produce a negative aggregate total even when the variable is present and working.

Root cause

The smoke test was using a positivity check where it really needed a nonzero-magnitude check.

Validation

• make format
• python -m py_compile policyengine_us/tests/microsimulation/test_microsim.py
• confirmed locally that MicroSeries.sum() can be negative while MicroSeries.abs().sum() remains nonzero

Notes

I could not run the full dataset-backed microsimulation test locally in this sandbox because the Hugging Face datasets are not reachable here, so CI should be the source of truth for the full repro and verification.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.84%. Comparing base (b70c547) to head (65e4304).
⚠️ Report is 13 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##              main    #7978      +/-   ##
===========================================
- Coverage   100.00%   92.84%   -7.16%     
===========================================
  Files            1     4212    +4211     
  Lines           13    60818   +60805     
  Branches         0      307     +307     
===========================================
+ Hits            13    56468   +56455     
- Misses           0     4222    +4222     
- Partials         0      128     +128     

Flag	Coverage Δ
unittests	92.84% <ø> (-7.16%)	⬇️

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[baogorek]

Please see the comment. My robot thinks self_employment_income can be negative whereas employment_income is non-negative, so they should be treated separately.

[baogorek]

My robot is saying that we should split these apart in the list, do .sum() > 0 for employment_income and .abs().sum() > 0 for self_employment_income