Limit Alabama FCEP eligibility by county

[daphnehanse11]

Fixes #8521.

Summary

• Adds an Alabama-only FCEP geography check for Montgomery, Macon, and Russell counties.
• Leaves non-Alabama FCEP states on the existing state-level availability path.
• Adds Alabama county regression cases for an eligible listed county and an ineligible unlisted county.

Tests

• uv run python -m policyengine_core.scripts.policyengine_command test policyengine_us/tests/policy/baseline/gov/hhs/chip/is_chip_eligible_fcep.yaml -c policyengine_us (12 passed)
• git diff --check

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (47cb26b) to head (3baeca1).
⚠️ Report is 24 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main     #8723   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            3         1    -2     
  Lines           47        25   -22     
=========================================
- Hits            47        25   -22     

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

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