Update Missouri CHIP premiums for July 2026 and fix tier boundary selection

[hua7450]

Summary

Updates Missouri's MO HealthNet for Kids CHIP premiums:

1. July 2026 schedule — adds the amounts effective July 1, 2026 from Appendix E, IM-4(PRM) (07-26), and extends the family-size brackets from 8 to 12 with full historical backfill (all four encoded chart vintages publish sizes 1–12; sizes 13+ take the size-12 amount, matching the chart's "13+ available upon request" cutoff).
2. Tier selection follows the chart's dollar boundaries — the formula reproduces Missouri's published methodology: each tier boundary is the FPL percentage converted to monthly dollars and rounded up to the next whole dollar, income at a published boundary falls in the lower tier, and both the boundaries and the premium row key on the CHIP child's MAGI household size, which counts a pregnant member as one person — the unborn child counts only in the pregnant member's own household (DSS manual 1885.010.00 example). The tier FPL fractions (1.50/1.85/2.25) live in premium/fpl_floor/tier_{1,2,3}.yaml.

Fixes #9129
Fixes #9130

Regulatory authority

• RSMo 208.640 — tier structure (>150–185%, >185–225%, >225–300% FPL) and premium percentages; 5% of gross income cap
• MAGI Manual 1840.025.00 — premium formula; defers to Appendix E, updated each July 1
• Appendix E chart — the operative dollar ranges

The ceiling rule is verified against all published boundaries (sizes 1–12) in the July 2025 and July 2026 charts (e.g., family of 2 at 185%: raw $3,336.17 → chart $3,337.00; family of 8 at 185% in 2025: raw $8,348.125 → chart $8,349.00, ruling out nearest-dollar rounding; exact whole dollars like $1,995.00 stay unchanged).

July 2026 premium amounts (monthly)

Family size	Tier 1 (>150–185% FPL)	Tier 2 (>185–225%)	Tier 3 (>225–300%)
1	$19	$62	$150
2	$25 (unchanged)	$83	$203
3	$32	$105	$256
4	$39	$127	$309
5	$45	$148	$363
6	$52	$170	$416
7	$58	$191	$469
8	$65	$214	$522
9	$72	$236	$576
10	$78	$257	$629
11	$85	$279	$682
12	$92	$301	$735

Behavior changes vs. main

• Households at a published boundary amount fall in the lower tier (e.g., family of 2 at exactly $3,337/month pays tier 1, not tier 2), and the tier 1 floor is inclusive at the ceiled dollar, matching the chart's range start.
• Families of 9+ pay the published larger-family amounts instead of the size-8 amount.
• A pregnant member's expected children no longer shift the chart's income boundaries; the chart keys on the CHIP child's MAGI household size, which excludes the unborn child.

Scope notes

• 5% cost-sharing cap: the federal chip_premium wrapper already applies the 42 CFR 457.560 / RSMo 208.640 cap downstream (min_ against medicaid_magi × gov.hhs.chip.cost_sharing.cap.rate), so mo_chip_premium intentionally stays gross.
• Known accepted simplification: the April 2020 chart started tier 1 one cent above the exact 150% FPL amount ($1,595.01 for one person) rather than at the ceiled dollar inclusive as the July 2024+ charts do; the formula applies the current inclusive rule to all years. This diverges from the 2020 chart only for incomes exactly at whole-dollar 150% FPL points in 2020–2021.
• Known annual-model limitation (pre-existing): PE resolves both the FPG and the premium amounts at January 1, so a calendar year pairs that year's FPL boundaries with the prior July's amounts; Missouri switches both mid-year.

Files

• parameters/gov/states/mo/hhs/chip/premium/tier_{1,2,3}.yaml — 2026-07-01 amounts, size 9–12 brackets
• parameters/gov/states/mo/hhs/chip/premium/fpl_floor/tier_{1,2,3}.yaml — tier FPL fractions
• variables/gov/states/mo/hhs/chip/mo_chip_premium.py — dollar-boundary tier selection keyed on MAGI household size
• tests/policy/baseline/gov/states/mo/hhs/chip/mo_chip_premium.yaml — 16 tests: 2027-period rate tests (incl. family of 10), boundary tests at the published family-of-2 dollar amounts, pregnancy tests

Test plan

• 2027-period tests for all three tiers plus a size-10 case at the July 2026 rates
• Boundary tests at $2,704/$2,705/$3,337/$3,338/$4,058/$4,059 monthly for a family of 2
• Pregnancy tests confirming a pregnant family of 2 keeps the size-2 chart
• CI passes

🤖 Generated with Claude Code

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (74f045a) to head (49aee33).
⚠️ Report is 9 commits behind head on main.

Additional details and impacted files

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