Model Medicare Part B premiums net of MSP coverage

[MaxGhenis]

Summary

• model baseline Medicare Part B premiums instead of treating them as a copied CPS input
• keep reported Part B premiums as a separate audit variable
• net out a cycle-free MSP proxy so likely MSP-covered standard premiums do not count toward baseline SPM MOOP

Testing

• uv run pytest -q policyengine_us/tests/policy/baseline/gov/hhs/medicare/test_part_b_msp_offset.py

[codecov]

Codecov Report

❌ Patch coverage is 85.33333% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.83%. Comparing base (b70c547) to head (96e9900).
⚠️ Report is 11 commits behind head on main.

Files with missing lines	Patch %	Lines
...usehold/expense/health/medicare_part_b_premiums.py	75.00%	6 Missing and 5 partials ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##              main    #7972      +/-   ##
===========================================
- Coverage   100.00%   90.83%   -9.17%     
===========================================
  Files            1        5       +4     
  Lines           13      120     +107     
  Branches         0       11      +11     
===========================================
+ Hits            13      109      +96     
- Misses           0        6       +6     
- Partials         0        5       +5     

Flag	Coverage Δ
unittests	90.83% <85.33%> (-9.17%)	⬇️

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