Make input variables pure leaves

[MaxGhenis]

Summary

• remove non-geographic defined_for gates from pure input variables
• make medicare_enrolled default to false so data must populate enrollment explicitly
• require NY Drive Clean rebate calculations to check the qualifying-purchase input directly
• fix il_aabd_institutional_status so its state gate is Illinois, not Massachusetts
• add code-health tests preventing input variables from using formulas, adds, subtracts, or non-geographic defined_for gates

Testing

• uv run ruff format ...
• uv run ruff check ...
• uv run pytest policyengine_us/tests/core/test_input_variable_definitions.py
• uv run policyengine-core test policyengine_us/tests/policy/baseline/gov/states/ny/nyserda/drive_clean_rebate.yaml -c policyengine_us
• Earlier targeted policy run covering Medicare, LA relief/DSS, WI Works, MA TAFDC, KY pension exclusion, SC rebate, and NY Drive Clean: 62 passed.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (20705b0) to head (847f023).
⚠️ Report is 29 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##             main     #8162       +/-   ##
============================================
+ Coverage   85.36%   100.00%   +14.63%     
============================================
  Files           3        10        +7     
  Lines          41        97       +56     
  Branches        2         0        -2     
============================================
+ Hits           35        97       +62     
+ Misses          6         0        -6     

Flag	Coverage Δ
unittests	100.00% <100.00%> (+14.63%)	⬆️

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