Support Medicaid MAGI claimant links

[daphnehanse11]

Summary

Fixes #8676.

This PR adds optional Medicaid MAGI claimant tax unit links so PE-US can represent dependents claimed by a tax unit outside their current modeled tax unit when data provide the link. When the claimant link is missing or does not match a modeled tax unit, PE-US keeps the existing conservative non-filer fallback.

Regulatory Authority

• 42 CFR 435.603(f)(2): Medicaid MAGI household rules for individuals expected to be claimed as tax dependents.
• 42 CFR 435.603(f)(3): non-filer household rules and dependent exceptions.

Changes

• Adds medicaid_claiming_tax_unit_id as an optional person-level input.
• Adds medicaid_has_known_claiming_tax_unit to validate that the claimant ID maps to a modeled tax unit.
• Routes Medicaid household size and income through the claimant tax household when the claimant is known.
• Includes externally claimed dependents in the claimant tax household.
• Adds medicaid_uses_missing_claimant_fallback and medicaid_missing_claimant_fallback_person_count diagnostics.
• Preserves non-filer fallback for claimed-elsewhere dependents without a known claimant link.

Requirements Coverage

Requirement	Coverage
Known outside claimant tax unit	Case 7
Unknown claimant fallback	Cases 3 and 8
Non-custodial-parent exception	Case 9
No-dependent case	Case 10
Missing-claimant diagnostics	Cases 3, 7, 8, and 9

Validation

• .venv/bin/ruff format .
• .venv/bin/ruff check .
• .venv/bin/policyengine-core test policyengine_us/tests/policy/baseline/gov/hhs/medicaid/income -c policyengine_us

Known Failing Tests

None.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (fa285ae) to head (8d174bb).
⚠️ Report is 420 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##              main     #8698    +/-   ##
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  Coverage   100.00%   100.00%            
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  Files            5         9     +4     
  Lines           62       162   +100     
  Branches         0         2     +2     
==========================================
+ Hits            62       162   +100     

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

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