Update Medicaid work requirement implementation

[daphnehanse11]

Summary:

• Scope Medicaid work requirement gating to the ACA adult Medicaid category instead of all Medicaid categories.
• Add currently modelable CMS/law exclusions and safe harbors: earned-income threshold, half-time student status, pregnant/postpartum medical assistance, former foster care under 26, Medicare eligibility, incarceration, and Indian Health Service coverage.
• Refresh parameter citations/descriptions and add targeted eligibility tests.

Audit notes:

• H.R. 1 became Public Law 119-21 on July 4, 2025.
• CMS published the Dec. 8, 2025 CMCS Informational Bulletin on Medicaid community engagement requirements.
• I did not find a published Medicaid community engagement interim final rule as of May 12, 2026.

Tests:

• uv run python policyengine_us/tests/test_batched.py policyengine_us/tests/policy/baseline/gov/hhs/medicaid --batches 1
• .venv/bin/ruff format --check .
• .venv/bin/ruff check .
• git diff --check upstream/main...HEAD
• GitHub Actions: all PR checks passing

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (623a65c) to head (4844f73).
⚠️ Report is 162 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##              main     #8257    +/-   ##
==========================================
  Coverage   100.00%   100.00%            
==========================================
  Files           12         2    -10     
  Lines          173        57   -116     
  Branches         4         1     -3     
==========================================
- Hits           173        57   -116     

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

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