Exclude DACA from general CHIP child and standard pregnancy eligibility

[hua7450]

Variables to amend

• policyengine_us/variables/gov/hhs/chip/is_chip_eligible_child.py
• policyengine_us/variables/gov/hhs/chip/is_chip_eligible_standard_pregnant_person.py

The general CHIP child and standard pregnant-person formulas currently exclude only ImmigrationStatus.UNDOCUMENTED:

undocumented = istatus == istatus.possible_values.UNDOCUMENTED
immigration_eligible = ~undocumented

Consequently, an applicant with immigration_status: DACA passes the immigration check and can receive modeled CHIP if the other eligibility conditions pass. DACA alone does not establish eligibility for these federally funded CHIP pathways.

Legal basis

• 8 U.S.C. § 1611(a) establishes the general restriction on federal public benefits for nonqualified noncitizens, subject to exceptions. 8 U.S.C. § 1641 defines qualified statuses; DACA alone is not one of them.
• CMS SHO #12-002 (August 28, 2012) explicitly excludes DACA recipients from Medicaid and CHIP eligibility under the CHIPRA section 214 lawfully residing state option. This is CMS guidance implementing the statutory framework, rather than statutory text naming DACA.
• CMS SHO #26-001 (April 8, 2026), page 28, footnote 9 reaffirms that DACA recipients are not eligible for full Medicaid and CHIP benefits under that option.

Proposed scope

In both formulas, exclude DACA alongside UNDOCUMENTED:

undocumented = istatus == istatus.possible_values.UNDOCUMENTED
daca = istatus == istatus.possible_values.DACA
immigration_eligible = ~(undocumented | daca)

The production logic change should require only these two existing formulas, with no new variables or parameters. Add focused regression cases to the existing YAML tests and the required changelog fragment.

Do not add this exclusion to is_chip_fcep_eligible_person: the FCEP pathway covers the unborn child regardless of the pregnant parent's immigration status. See CMS FCEP eligibility rules.

This issue is limited to the missing DACA exclusion. State elections, five-year waiting-period rules, and other immigration eligibility gaps are separate work.

Acceptance criteria

• An otherwise eligible DACA child is ineligible through is_chip_eligible_child.
• An otherwise eligible DACA pregnant applicant is ineligible through is_chip_eligible_standard_pregnant_person in a state offering that pathway.
• Existing undocumented exclusions and otherwise eligible citizen cases remain correct.
• FCEP remains available to otherwise eligible pregnancies regardless of whether the pregnant parent is undocumented or has DACA.
• Extend the existing is_chip_eligible_child.yaml and is_chip_eligible_standard_pregnant_person.yaml policy tests, keeping the fix narrowly scoped.

Found while reviewing #9434. That PR's CA CCHIP formula already excludes both statuses; this is a separate gap in the shared CHIP formulas. Diagnosis is based on code inspection; no new local tests were run to file this issue.