Correct Missouri Medicaid child income limit to 153% of FPL

[daphnehanse11]

Fixes #9126.

Change

Corrects Missouri's Medicaid income limit for children ages 1-18 from 1.55 to 1.53 in both the young_child and older_child parameters.

Missouri's MO HealthNet for Kids base standard is 148% of FPL for all children ages 1-18 (DSS Manual § 1830.010.05), and the conditional 5% MAGI disregard (§ 1805.030.20.20.05) produces an effective 153% ceiling. The state's published dollar limits equal exactly 148% of the FPL ($23,621 for one person at the 04/2026 guidelines).

The prior 1.55 traces to KFF's figure, which reflects the pre-MAGI-conversion 150% standard (+5). Since the 148% MAGI-converted standard predates the parameter's 2018 start date, the value is corrected at the existing date rather than dated as a policy change. KFF reports 155% for January 2018, 2021, and 2025 alike, so this is a source discrepancy, not a recent change — a comment in each parameter file documents this.

Impact

• Children in the 153-155% FPL band were incorrectly shown Medicaid-eligible (confirmed by partner testing for one- and two-person households).
• The same children were misrouted away from CHIP, since is_chip_eligible_child requires non-Medicaid-eligibility before applying the CHIP ceiling (MO: 305%).

Tests

• Adds four boundary regression tests (152% eligible / 154% ineligible, for both age groups).
• Full Medicaid suite (380), CHIP + Missouri state suites (626), and partner contract suite (630) all pass; partner tests untouched.

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