Retire legacy Marketplace coverage input

[daphnehanse11]

Fixes #8674

Summary

This PR retires the legacy Marketplace coverage input from the active input/export surface while preserving the variable name as a compatibility alias. It also canonicalizes household health benefit accounting on annual health-value variables, including assigned_aca_ptc rather than the monthly premium_tax_credit wrapper.

API partner implications

This is intentionally API-surface-relevant:

• has_marketplace_health_coverage is no longer an active input variable discovered from PE-US metadata.
• The legacy variable remains calculable and now mirrors has_marketplace_health_coverage_at_interview.
• Partners exporting PE-US input variables should stop surfacing has_marketplace_health_coverage.
• Reported Marketplace coverage should use has_marketplace_health_coverage_at_interview.
• Simulated ACA take-up should use takes_up_aca_if_eligible.
• Annual health resource accounting should use healthcare_benefit_value / assigned_aca_ptc; premium_tax_credit remains a monthly wrapper.

Health accounting semantics

• healthcare_benefit_value: annual canonical household resource value of health coverage.
• household_health_benefits: annual health value included in household_benefits only when gov.simulation.include_health_benefits_in_net_income is enabled.
• Program wrappers medicaid, chip, and monthly premium_tax_credit are no longer listed in household health benefits to avoid double-counting or period mismatch.

Tests

• uv run policyengine-core test policyengine_us/tests/policy/baseline/household/household_health_benefits.yaml policyengine_us/tests/policy/baseline/household/healthcare_benefit_value.yaml policyengine_us/tests/policy/baseline/household/expense/health/has_marketplace_health_coverage.yaml policyengine_us/tests/policy/baseline/gov/aca/ptc/premium_tax_credit.yaml policyengine_us/tests/policy/baseline/gov/aca/eligibility/is_aca_ptc_eligible.yaml policyengine_us/tests/policy/baseline/gov/aca/ptc/aca_ptc.yaml -c policyengine_us
• uv run pytest policyengine_us/tests/core/test_input_variable_definitions.py -q
• uv run policyengine-core test policyengine_us/tests/policy/baseline/household/household_health_benefits.yaml -c policyengine_us
• make format
• git diff --check

CI follow-up

An initial CI changelog check failed because the fragment used the old changelog.d/fixed/8674.md path. This branch now uses changelog.d/8674.fixed.md, matching the current PR workflow.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (f9e58e7) to head (2b81b22).
⚠️ Report is 161 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main     #8693   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files            3         3           
  Lines           55        31   -24     
  Branches         0         1    +1     
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- Hits            55        31   -24     

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

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[MaxGhenis]

Approving — a clean soft-retirement plus health-accounting canonicalization, well-guarded by tests (agent-assisted review; I verified the load-bearing parts).

Nothing in the model reads has_marketplace_health_coverage: it keeps its name, drops default_value = True, and now aliases has_marketplace_health_coverage_at_interview via a formula, so it stops being an input variable but stays calculable (set-inputs still override). The effective default flips True→False, but with zero formula readers that's inert. #8187 already pulled it out of the PTC path; this finishes the retirement.

The household_health_benefits parameter lands on the same 7-variable list as healthcare_benefit_value: medicaid→medicaid_cost and premium_tax_credit→assigned_aca_ptc are value-neutral, and the real delta — picking up co_omnisalud and or_healthier_oregon_cost — only applies when include_health_benefits_in_net_income is on (default off). No double-count: Healthier Oregon is gated on federally-barred-non-citizen status (disjoint from medicaid_cost) and OmniSalud enrollees have no federal APTC (disjoint from assigned_aca_ptc). Case 4 nails the double-count guard. fixed is the right fragment type.

Two non-blocking notes:

• The canonical list now lives in two hand-kept places (healthcare_benefit_value.adds and the parameter). That's the drift #8674 wanted to kill — consider making household_health_benefits gate healthcare_benefit_value directly unless you're intentionally keeping the set reform-modifiable via the parameter.
• Downstream follow-up lives in populace: it still carries has_marketplace_health_coverage in cps_carried.py and asserts on it in test_us_puf_support.py. Nothing breaks, but the export surface should switch to the _at_interview variable to finish the retirement there.