Fix MAGI Medicaid household construction

[daphnehanse11]

Fixes #5907

What this does

• replace the MAGI Medicaid tax-unit proxy with person-specific tax-filer and non-filer household rules
• exclude child and dependent income only when it is below the Medicaid filing-threshold rule
• add targeted Medicaid household tests and make affected ACA fixtures explicit about parent-child relationships
• model the full-time-student-under-21 household rule as an explicit state option switch instead of a faux default threshold

Known gaps

• the non-custodial-parent exception still needs custody data
• dependents claimed elsewhere still do not have claimant-tax-unit linkage
• state-by-state backfill is still needed for the full-time-student-under-21 option
• pregnancy treatment for other household members remains unparameterized

Validation

• uv run --with ruff ruff format .
• uv run --with ruff ruff check .
• uv run python -m policyengine_core.scripts.policyengine_command test policyengine_us/tests/policy/baseline/gov/hhs/medicaid/income/medicaid_uses_non_filer_rules.yaml policyengine_us/tests/policy/baseline/gov/hhs/medicaid/income/medicaid_income_level.yaml policyengine_us/tests/policy/baseline/gov/hhs/medicaid/income/tax_unit_medicaid_income_level.yaml policyengine_us/tests/policy/baseline/gov/hhs/chip policyengine_us/tests/policy/baseline/gov/aca/eligibility/aca_magi.yaml policyengine_us/tests/policy/baseline/gov/aca/ptc/integration.yaml policyengine_us/tests/policy/baseline/gov/aca/ptc/separate_esi.yaml -c policyengine_us

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (f9a4a0f) to head (98d472a).
⚠️ Report is 1154 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main     #8169   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            7        16    +9     
  Lines          215       224    +9     
  Branches         6         2    -4     
=========================================
+ Hits           215       224    +9     

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

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[daphnehanse11]

Initial read-only review via gh/PR diff before edits.

What I checked:

• Person-specific Medicaid MAGI household size/income rules against the tax-filer, tax-dependent-exception, and non-filer household structure.
• Child/dependent filing-threshold income exclusion path and the new Medicaid tests.
• Variable metadata, parameter metadata, period handling, and changed healthcare/ACA test expectations.

Findings I am patching in a fresh /tmp worktree:

• medicaid_non_filer_child_age_eligible parameterizes the under-19 non-student child limit but still hardcodes the under-21 full-time-student option. I will add a student age-limit parameter so both legal thresholds live in parameters.
• medicaid_person_is_required_to_file hardcodes the $5 gross-income threshold for married dependents whose spouse itemizes. I will parameterize that IRS filing-rule threshold instead of leaving a legal dollar amount in formula code.

I do not see a broader blocker in the submitted diff yet; these look like focused PolicyEngine-style fixes before merge.

[daphnehanse11]

Follow-up after the initial patch: Codecov was still flagging missing patch coverage in medicaid_adjusted_gross_income_person.py, so I added a focused Medicaid AGI-person test covering the single-vs-joint shared ALD split, taxable basic income handling, and the dependent/no-shared-allocation path.

Local verification:

• policyengine_command test policyengine_us/tests/policy/baseline/gov/hhs/medicaid/income/medicaid_adjusted_gross_income_person.yaml -c policyengine_us: 2 passed
• policyengine_command test policyengine_us/tests/policy/baseline/gov/hhs/medicaid/income -c policyengine_us: 19 passed
• local coverage for medicaid_adjusted_gross_income_person.py: 100% line and branch coverage
• ruff check on the modified Medicaid Python files: passed
• git diff --check: passed

Pushed follow-up commit: 98d472a78d.