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 (92fdab8) to head (597db48).
⚠️ Report is 82 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main     #8433   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            2         2           
  Lines           57        41   -16     
  Branches         1         0    -1     
=========================================
- Hits            57        41   -16     

Flag	Coverage Δ
unittests	100.00% <100.00%> (ø)

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