Add person_receives_aca and assigned ACA PTC variables

[daphnehanse11]

Closes #7806

Summary

• add person_receives_aca as a person-level ACA receipt flag
• keep aca_ptc as the pre-takeup ACA premium tax credit amount and add assigned_aca_ptc for the amount actually assigned after take-up
• add YAML coverage for base ACA PTC behavior, assigned ACA PTC behavior, and person-level ACA receipt

Testing

• make format
• uv run policyengine-core test policyengine_us/tests/policy/baseline/gov/aca/ptc/aca_ptc.yaml policyengine_us/tests/policy/baseline/gov/aca/ptc/assigned_aca_ptc.yaml policyengine_us/tests/policy/baseline/gov/aca/person_receives_aca.yaml -c policyengine_us

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (752fae4) to head (9aa0f7a).
⚠️ Report is 118 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main     #7808   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            2         3    +1     
  Lines           13        35   +22     
=========================================
+ Hits            13        35   +22     

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

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.