Fix MSP spouse deeming and resource scoping

[daphnehanse11]

Summary

• apply spouse deeming in MSP countable income and compare married applicants using spouse-aware income
• scope MSP resources to the applicant and spouse instead of the full SPM unit, and align MSP eligibility with the resolved MSP category
• tighten MSP category boundary tests and backfill Medicare premium history used by MSP benefit coverage

Testing

• uv run --extra dev ruff format .
• uv run --extra dev ruff check policyengine_us/variables/gov/hhs/medicare/savings_programs policyengine_us/parameters/gov/hhs/medicare/savings_programs policyengine_us/tests/policy/baseline/gov/hhs/medicare/savings_programs
• uv run policyengine-core test policyengine_us/tests/policy/baseline/gov/hhs/medicare/savings_programs -c policyengine_us
• uv run --extra dev ruff check policyengine_us/parameters/gov/hhs/medicare policyengine_us/tests/policy/baseline/gov/hhs/medicare changelog.d/codex-msp-ssi-deeming-assets.fixed.md
• uv run policyengine-core test policyengine_us/tests/policy/baseline/gov/hhs/medicare/eligibility/base_part_a_premium.yaml policyengine_us/tests/policy/baseline/gov/hhs/medicare/eligibility/base_part_b_premium.yaml policyengine_us/tests/policy/baseline/gov/hhs/medicare/savings_programs/msp_benefit_value.yaml -c policyengine_us

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (b527c7e) to head (5c1030f).
⚠️ Report is 19 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##              main     #7893    +/-   ##
==========================================
  Coverage   100.00%   100.00%            
==========================================
  Files            3         9     +6     
  Lines           43       159   +116     
==========================================
+ Hits            43       159   +116     

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

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