Include Medicare Part A premiums and Part D IRMAA in MOOP

[daphnehanse11]

Summary

• add a net medicare_part_a_premium variable for Part A premiums paid out of pocket
• add msp_part_a_premium_coverage so QMB-covered Part A premiums do not count as household MOOP
• include paid Part A premiums and Part D IRMAA in SPM health insurance premiums and medical out-of-pocket expenses

Fixes #8529.

Tests

uv run python -m policyengine_core.scripts.policyengine_command test -c policyengine_us policyengine_us/tests/policy/baseline/gov/hhs/medicare/eligibility/medicare_part_a_premium.yaml policyengine_us/tests/policy/baseline/gov/hhs/medicare/savings_programs/msp_part_a_premium_coverage.yaml policyengine_us/tests/policy/baseline/household/income/spm_unit/spm_unit_medical_out_of_pocket_expenses_medicare_part_b.yaml

uv run python -m policyengine_core.scripts.policyengine_command test -c policyengine_us policyengine_us/tests/policy/baseline/gov/hhs/medicare policyengine_us/tests/policy/baseline/household/income/spm_unit/spm_unit_medical_out_of_pocket_expenses_medicare_part_b.yaml

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (7f028e9) to head (44c1b50).
⚠️ Report is 9 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main     #8532   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files            2         4    +2     
  Lines           30        49   +19     
  Branches         0         1    +1     
=========================================
+ Hits            30        49   +19     

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

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[daphnehanse11]

Scope mismatch: this PR says it includes Medicare Part D premiums in MOOP, but spm_unit_health_insurance_premiums only adds income_adjusted_part_d_premium_surcharge. The surcharge variable documentation correctly says this is paid in addition to the beneficiary's underlying Part D plan premium, and I do not see a modeled base Part D premium or Part D enrollment variable in the PR. As written, ordinary Part D premiums remain omitted from MOOP, and high-income beneficiaries only get the IRMAA surcharge. Please add the base Part D premium path, or narrow the PR/docs/tests to say this only models Part D IRMAA.