Use WIC pregnancy count in family size

[MaxGhenis]

Summary

• use summed current_pregnancies instead of a boolean pregnancy flag for WIC family-size adjustments
• preserve the is_pregnant-only path because current_pregnancies defaults from is_pregnant
• add WIC FPG and income-limit regression tests for twin pregnancy

Fixes #8186.

Tests

• uv run python -m policyengine_core.scripts.policyengine_command test policyengine_us/tests/policy/baseline/gov/usda/wic/wic_fpg.yaml policyengine_us/tests/policy/baseline/gov/usda/wic/wic_income_limit.yaml -c policyengine_us
• uv run ruff check policyengine_us/variables/gov/usda/wic/wic_fpg.py policyengine_us/variables/gov/usda/wic/wic_income_limit.py
• git diff --check

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (bd1ae42) to head (534cf7b).
⚠️ Report is 18 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##             main     #8433       +/-   ##
============================================
+ Coverage   77.77%   100.00%   +22.22%     
============================================
  Files           1         2        +1     
  Lines           9        41       +32     
============================================
+ Hits            7        41       +34     
+ Misses          2         0        -2     

Flag	Coverage Δ
unittests	100.00% <100.00%> (+22.22%)	⬆️

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