Fix ACA family tier premiums for older dependents

[MaxGhenis]

Summary

• Count premium-paying ACA tax dependents under age 26 as dependent children for family-tier states when there is a covered adult family-tier anchor.
• Treat New York tax-dependent children ages 26 through 29 as dependent children using the NY Age 29 child tier rate, while avoiding NY Child Only pricing for covered age-26-through-29 dependents without a covered adult.
• Add extra adult benchmark premiums when NY/VT tax units have more than two premium-paying adults, including older adult dependents.
• Apply the same family-tier logic to SLCSP and lowest-cost bronze plan premiums.

Policy Sources

• Federal PTC uses the taxpayer, spouse, and tax dependents as the relevant tax family/coverage family: https://uscode.house.gov/quicksearch/get.plx?section=36B&title=26
• NY State of Health SLCSP guidance maps an adult child claimed as a tax dependent to Couple + Child(ren) and the NY SLCSP rate tables separately publish Dependent Children Under Age 26 and Dependent Children Ages 26-29 columns: https://info.nystateofhealth.ny.gov/sites/default/files/2024%20FAQs%20on%20SLCSP%20English.pdf and https://info.nystateofhealth.ny.gov/sites/default/files/2025%20SLCSP%20Rates%20by%20County.FINAL.pdf
• New York's Age 29 dependent coverage authority is in Insurance Law Section 3216: https://www.nysenate.gov/legislation/laws/ISC/3216
• Vermont uses family rating tiers and treats ordinary dependent children as children through age 25; 26+ dependent-child coverage is limited to incapacitated dependents, which the model does not currently represent: https://info.healthconnect.vermont.gov/sites/vhc/files/documents/2025%20Catastrophic_Plans.pdf

Root Cause

NY and VT use family-tier premiums instead of age curves, but the model reused the under-21 age-rating child cutoff and collapsed all adult counts above two into the two-adult tier. That treated 21-25 tax dependents as adults, missed NY's separate age-26-through-29 child tier, and dropped premium costs for older adult dependents.

Review Notes

A secondary review found two issues. This revision fixes the Child Only edge case for NY age-26-through-29 dependents without a covered adult and avoids applying the NY Age 29 child factor to extra older-adult premiums. The remaining precision limitation is that NYSOH publishes exact SLCSP amounts by county and family type, while the current model stores NY benchmark inputs by rating area/base adult premium; this PR keeps the existing family-tier multiplier approach and adds the Age 29 factor within that framework.

Tests

• uv run ruff check policyengine_us/variables/gov/aca/slspc/is_aca_family_tier_dependent_child.py policyengine_us/variables/gov/aca/slspc/is_aca_ny_age_29_dependent_child.py policyengine_us/variables/gov/aca/slspc/slcsp_family_tier_category.py policyengine_us/variables/gov/aca/lcbp/lcbp_family_tier_category.py policyengine_us/variables/gov/aca/slspc/slcsp_family_tier_multiplier.py policyengine_us/variables/gov/aca/lcbp/lcbp_family_tier_multiplier.py
• uv run python -m policyengine_core.scripts.policyengine_command test policyengine_us/tests/policy/baseline/gov/aca/slcsp/family_tier_category.yaml -c policyengine_us
• uv run python -m policyengine_core.scripts.policyengine_command test policyengine_us/tests/policy/baseline/gov/aca/slcsp/family_tier_integration.yaml -c policyengine_us
• uv run python -m policyengine_core.scripts.policyengine_command test policyengine_us/tests/policy/baseline/gov/aca/lcbp/lcbp_family_tier_amount.yaml -c policyengine_us
• uv run python -m policyengine_core.scripts.policyengine_command test policyengine_us/tests/policy/baseline/gov/aca -c policyengine_us

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (623a65c) to head (aa16a7e).
⚠️ Report is 136 commits behind head on main.

Additional details and impacted files

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##              main     #8238   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files           12         6    -6     
  Lines          173       148   -25     
  Branches         4         0    -4     
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- Hits           173       148   -25     

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

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