Make CHIP health benefit value respect enrollment

[daphnehanse11]

Fixes #8673.

Summary

• Change healthcare_benefit_value to include CHIP through chip, which is gated by chip_enrolled.
• Document CHIP net/gross and eligibility/enrollment semantics on the existing CHIP value and cost variables.
• Add healthcare benefit value tests for CHIP enrolled, CHIP eligible but not enrolled, and CHIP ineligible cases.

CHIP Semantics

• chip: enrolled, take-up-aware, net CHIP benefit value.
• per_capita_chip: eligibility-gated per-capita net CHIP payment; not a household resource accounting output by itself.
• chip_gross and per_capita_chip_gross: eligibility-gated gross value concepts, kept separate from household health benefit value.
• chip_federal_cost and chip_state_cost: enrollment-gated budget cost variables through chip.

Validation

• uv run policyengine-core test policyengine_us/tests/policy/baseline/household/healthcare_benefit_value.yaml -c policyengine_us: 5 passed, 1 warning.
• uv run policyengine-core test policyengine_us/tests/policy/baseline/gov/hhs/chip -c policyengine_us: 70 passed, 1 warning.
• uv run policyengine-core test policyengine_us/tests/policy/baseline/household/household_health_benefits.yaml -c policyengine_us: 3 passed, 1 warning.
• make format: passed.
• git diff --check: passed.

Known Failing Tests

None.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (206b9e7) to head (dc5ce0b).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main     #8679   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
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  Files            8         7    -1     
  Lines          127        97   -30     
=========================================
- Hits           127        97   -30     

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

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