Add chip_premium with 17-state CHIP premium implementations

[MaxGhenis]

Fixes #5889.

Summary

Adds chip_premium — a federal tax-unit variable that aggregates state-specific CHIP premium and enrollment-fee implementations — along with concrete implementations for 17 states:

State	Structure	Source
AL	Per-child annual enrollment fee, family cap	Alabama ALL Kids Premiums and Copays
CT	Family flat monthly, Band 2 only	HUSKY Health Monthly Income Chart 3/1/2025
DE	Family flat monthly, two tiers	Delaware Healthy Children Program
FL	Family flat monthly, two tiers	Florida KidCare 2025 Income Guidelines
GA	Per-child monthly with cap, six FPL tiers	Georgia DFCS Medicaid Manual 2194
IA	Per-child monthly with cap, two tiers	Iowa Hawki 2025 Income Guidelines
ID	Per-child monthly, no family cap	Idaho CHIP Title XXI State Plan
IL	Per-child monthly with cap, two tiers	Illinois HFS All Kids
IN	One-child vs two-or-more-children monthly rates, four tiers	Indiana Hoosier Healthwise Package C
KS	Family flat monthly, three tiers	Kansas Legislative Briefing Book 2026
LA	Family flat monthly, LaCHIP Affordable Plan only	LaCHIP Affordable Plan FAQ
MA	Per-child monthly with 3-child cap	130 CMR 506.011
MI	Family flat monthly, single tier	MIChild general information
MO	Family size × FPL tier monthly schedule	Missouri DSS Appendix E
NY	Per-child monthly with 3-child cap	NYSDOH Child Health Plus
TX	Annual enrollment fee, three tiers	Texas Works Handbook D-1820
WI	Per-child monthly, no cap, eleven tiers	BadgerCare Plus Handbook 48.1

Federal-level chip_premium defaults to zero and aggregates each state's contribution via adds, so adding or updating a state is isolated to its own files.

Architecture

• Tax-unit entity — CHIP premiums and enrollment fees are charged once per household.
• Income basis — all tiers use tax_unit_medicaid_income_level (MAGI / FPL), matching existing CHIP eligibility.
• Child count — per-child states sum is_chip_eligible_child across tax unit members and apply the family cap where applicable.
• Effective dates — parameter files carry the dates supported by each state's published schedule.

Not included

States confirmed as charging premiums per KFF May 2024 but deferred:

• NV — FPL tier boundaries for the $25/$50/$80 quarterly premium are not published on primary state sites.
• PA — premium varies by chosen MCO, which is out of scope without an MCO selection variable.
• ND, NJ — current status could not be verified from primary sources (NJ appears suspended; ND likely copays-only but not confirmed).

Also, the federal 5% of family income cost-sharing cap under 42 CFR 457.560 is cited in the federal variable but not enforced as a ceiling. It only binds at the top of CHIP income bands and can be added when premium + copay modeling is broader.

Testing

• 47 state-level YAML tests covering each implemented state (zero case, one or more premium tiers, and family-cap behavior where relevant).
• 4 federal aggregation tests: default-zero state, Texas enrollment fee, New York per-child premium, and Missouri family-size-tier premium.
• 86/86 CHIP tests pass. Existing non-premium CHIP tests unaffected.
• make format / ruff check clean.

Test plan

• Full CHIP test suite passes locally (86/86)
• Formatter / linter pass
• CI passes

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (05d5d97) to head (62b249f).
⚠️ Report is 21 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main     #8086   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           18        18           
  Lines          353       295   -58     
  Branches         4         0    -4     
=========================================
- Hits           353       295   -58     

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.