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 (47cb26b) to head (7c511ae).
⚠️ Report is 13 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main     #8693   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            3         3           
  Lines           47        31   -16     
  Branches         0         1    +1     
=========================================
- Hits            47        31   -16     

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

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