Add Medicaid waiver premium rules

[daphnehanse11]

Fixes #8102.

Summary

• Adds Michigan Healthy Michigan Plan contribution variables and parameters, with current-law collection disabled after the MI Health Account ended in 2024.
• Adds Montana HELP premium variables and parameters, with current-law collection disabled after premiums ceased on 2023-01-01.
• Wires both state variables into the federal medicaid_premium aggregator and covers the SPM MOOP wrapper path.
• Adds focused state, federal aggregator, SPM MOOP, and changelog coverage.

Sources

• Michigan Healthy Michigan Plan program changes: https://www.michigan.gov/healthymiplan
• Michigan Healthy Michigan waiver account contribution rules: https://www.medicaid.gov/Medicaid-CHIP-Program-Information/By-Topics/Waivers/1115/downloads/mi/Healthy-Michigan/mi-healthy-michigan-waiver-amend-req-11082013.pdf#page=13
• Montana HELP premium rules: https://www.medicaid.gov/Medicaid-CHIP-Program-Information/By-Topics/Waivers/1115/downloads/mt/HELP-program/mt-HELP-program-help-ops-prtcl-03012016.pdf#page=2
• Montana premium cessation: https://dphhs.mt.gov/HELPPlan/MT2022helpExpanQ4-annNarrB.pdf#page=4

Tests

• uv run --extra dev ruff format policyengine_us/variables/gov/hhs/medicaid/medicaid_premium.py policyengine_us/variables/gov/states/mi/mdhhs/healthy_michigan/is_mi_healthy_michigan_adult.py policyengine_us/variables/gov/states/mi/mdhhs/healthy_michigan/mi_healthy_michigan_contribution.py policyengine_us/variables/gov/states/mt/dphhs/help/is_mt_help_adult.py policyengine_us/variables/gov/states/mt/dphhs/help/mt_help_premium.py
• uv run python -m policyengine_core.scripts.policyengine_command test policyengine_us/tests/policy/baseline/gov/states/mi/mdhhs/healthy_michigan/mi_healthy_michigan_contribution.yaml policyengine_us/tests/policy/baseline/gov/states/mt/dphhs/help/mt_help_premium.yaml policyengine_us/tests/policy/baseline/gov/hhs/medicaid/medicaid_premium.yaml policyengine_us/tests/policy/baseline/household/income/spm_unit/spm_unit_medical_out_of_pocket_expenses.yaml -c policyengine_us
• uv run pytest policyengine_us/tests/test_parameter_files.py -q
• uv run --extra dev ruff check .

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (d888043) to head (7d6b17b).
⚠️ Report is 8 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##             main     #8179       +/-   ##
============================================
+ Coverage   86.04%   100.00%   +13.95%     
============================================
  Files           3         5        +2     
  Lines          43        68       +25     
  Branches        2         2               
============================================
+ Hits           37        68       +31     
+ Misses          6         0        -6     

Flag	Coverage Δ
unittests	100.00% <100.00%> (+13.95%)	⬆️

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.

[daphnehanse11]

Reviewed the PR diff and the cited MI/MT waiver materials before editing.

Findings I’m going to patch in a fresh worktree:

• The MI/MT state adult helper variables currently only test the federal adult age range. That lets Healthy Michigan / HELP premiums apply to people who are not in the expansion adult category, including pregnant people, even though the cited materials exempt pregnant people and describe these rules as applying to the adult expansion population.
• Montana’s lower premium threshold is modeled as income_level > 0.5, and the test expects a person exactly at 50% FPL to owe no premium. The Montana rule exempts people with MAGI under 50% FPL, so the boundary should include exactly 50% FPL.

Parameter metadata, references, annual/monthly period use, Medicaid premium aggregation, and SPM MOOP wiring otherwise looked structurally sound on this pass. I’m patching the above from a fresh detached worktree off origin/codex/issue-8102-medicaid-premiums.

[daphnehanse11]

Code-quality pass pushed in 6ee5913. I replaced boolean member-count checks with tax_unit.any(...), and made the premium formulas name medicaid_magi explicitly before applying the state rate. Focused Medicaid waiver premium/SPM tests pass locally: 16 passed.