Fix NY Child Health Plus full-premium tier

[daphnehanse11]

Summary

• Fill New York Child Health Plus premiums above 400% FPL with statewide full-premium PMPM proxies from CHPlus rate-development decks
• Preserve the existing three-child family cap for the representative full-premium tier
• Add a regression test for the >400% FPL case

Fixes #8097.

Tests

• uv run python -m policyengine_core.scripts.policyengine_command test policyengine_us/tests/policy/baseline/gov/states/ny/hhs/chip/ny_chip_premium.yaml -c policyengine_us
• uv run python -m policyengine_core.scripts.policyengine_command test policyengine_us/tests/policy/baseline/gov/hhs/chip/chip_premium.yaml -c policyengine_us
• uv run pytest policyengine_us/tests/test_parameter_files.py -q
• uv run --extra dev ruff check policyengine_us/variables/gov/states/ny/hhs/chip/ny_chip_premium.py
• uv run --extra dev ruff format --check policyengine_us/variables/gov/states/ny/hhs/chip/ny_chip_premium.py
• git diff --check

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (eee0f91) to head (138468d).
⚠️ Report is 60 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##             main     #8342       +/-   ##
============================================
+ Coverage   78.72%   100.00%   +21.27%     
============================================
  Files        4723         1     -4722     
  Lines       68774        18    -68756     
  Branches      340         0      -340     
============================================
- Hits        54142        18    -54124     
+ Misses      14554         0    -14554     
+ Partials       78         0       -78     

Flag	Coverage Δ
unittests	100.00% <ø> (+21.27%)	⬆️

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.