Use rules-based Medicare Part B in SPM medical out-of-pocket expenses

[MaxGhenis]

Closes half of #8095.

Context

PolicyEngine-US already has a rules-based Medicare Part B premium variable: income_adjusted_part_b_premium (base premium + IRMAA surcharge by MAGI from 2 years prior, per filing status). It lives in `variables/gov/hhs/medicare/eligibility/part_b/`.

But it's currently orphaned — nothing consumes it. The imputed `medicare_part_b_premiums` is what flows into `medical_out_of_pocket_expenses` via `health_insurance_premiums`.

Fix

In `spm_unit_medical_out_of_pocket_expenses` (the SPM-unit-level wrapper introduced by #8103), subtract the imputed figure and add the rules-based one:

```python
return (
imputed_moop
- imputed_medicare_part_b_premiums
+ computed_income_adjusted_part_b_premium
+ chip_premium
)
```

Baseline SPM now uses statute-driven Part B premium rather than CPS-imputed (removes per-family CPS noise). Reforms to the base premium or IRMAA thresholds propagate through SPM poverty measurement — previously they affected computed `income_adjusted_part_b_premium` but that variable was unused.

Person-level `medical_out_of_pocket_expenses` is untouched, so state itemized medical deductions and SNAP excess medical continue to see the imputed CPS figure.

Testing

• 2 new tests in `spm_unit_medical_out_of_pocket_expenses_medicare_part_b.yaml`: base-premium scenario and IRMAA surcharge scenario
• Existing SPM wrapper tests unaffected
• 6/6 SPM-MOOP tests pass locally

Caveats

This does not yet touch the Person-level `medical_out_of_pocket_expenses`. Doing so would swap the imputed figure for rules-based across all consumers (state deductions, SNAP, etc.), which is a larger change and warrants a separate PR once we confirm the rules-based figure matches CMS-aggregate calibration for baseline.

Test plan

• Local tests pass
• `make format` / `ruff check` clean
• CI passes

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (05d5d97) to head (cbc6454).
⚠️ Report is 37 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##              main     #8104    +/-   ##
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  Coverage   100.00%   100.00%            
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  Files           18         1    -17     
  Lines          353        14   -339     
  Branches         4         0     -4     
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- Hits           353        14   -339     

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unittests	100.00% <100.00%> (ø)

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