[codex] Add Missouri CSFP county filtering

[daphnehanse11]

Summary

• Adds mo_dhss_csfp_county_eligible from the Missouri DHSS CSFP site layer.
• Gates federal CSFP eligibility by the Missouri county variable when state_code is MO.
• Adds direct county tests and integrated CSFP eligibility tests for the Missouri launch nuance.

Closes #8581.

Validation

• uv run --extra dev ruff check policyengine_us/variables/gov/states/mo/dhss/csfp/mo_dhss_csfp_county_eligible.py policyengine_us/variables/gov/usda/csfp/commodity_supplemental_food_program_eligible.py
• uv run python -m policyengine_core.scripts.policyengine_command test policyengine_us/tests/policy/baseline/gov/states/mo/dhss/csfp/mo_dhss_csfp_county_eligible.yaml -c policyengine_us
• uv run python -m policyengine_core.scripts.policyengine_command test policyengine_us/tests/policy/baseline/gov/usda/csfp/commodity_supplemental_food_program_eligible.yaml -c policyengine_us

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (17cda0d) to head (d37024b).
⚠️ Report is 25 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main     #8584   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            1         2    +1     
  Lines           19        32   +13     
=========================================
+ Hits            19        32   +13     

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

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.