Add SNAP ABAWD foster youth (pre-HR1) and IHCIA Indian (post-HR1) exemptions

[daphnehanse11]

Fixes #8864
Fixes #8865

What this does

#8864 — Former foster youth exemption (pre-HR1 only)

The Fiscal Responsibility Act of 2023 added an ABAWD exception for individuals aged 24 or younger who were in foster care under State responsibility on their 18th birthday — 7 CFR 273.24(c)(9), in force September 2023 until removed by P.L. 119-21 (OBBBA) effective 2025-07-04 (confirmed removed by the FNS ABAWD Exceptions Implementation Memorandum, Oct 3, 2025).

• Reuses the existing was_in_foster_care person-level boolean input (default false), mirroring the Medicaid work requirement implementation (medicaid_work_requirement_eligible), which uses the same input for the OBBBA former foster care exemption.
• Adds parameter gov/usda/snap/work_requirements/abawd/age_threshold/former_foster_care.yaml (24, effective 2023-09-01), read via the existing parameters("2025-06-01") pre-HR1 snapshot so it only applies in the pre-HR1 branch (including CA's delayed adoption through 2026-05-31).
• ORed into the pre-HR1 branch alongside the homeless and veteran exemptions, following the existing ungated pattern for the other FRA 2023 exceptions.

#8865 — Indian, Urban Indian, and California Indian exemption (post-HR1 only)

The rules hook for 7 U.S.C. 2015(o)(3)(F)–(G) (P.L. 119-21, 139 STAT. 81-82; IHCIA definitions at 25 U.S.C. 1603(13), (28) and 1679(a)) already landed in #8372: is_snap_abawd_indian_exempt is ORed into the post-HR1 conditions only, with tests covering post-HR1 exemption and pre-HR1 no-effect (Cases 24–25). This PR completes the issue by documenting the input as a data-layer gap: survey data (CPS) have no tribal enrollment measure, so the input defaults to false pending populace imputation, and remains settable in household simulations.

Data dependency

Both inputs are data-layer gaps: survey identification of former foster youth and IHCIA Indian status is populace work. The rules hooks exist regardless for calculator use and an explicit assumption surface.

Tests

New Cases 33–35 in meets_snap_abawd_work_requirements.yaml:

• Former foster youth aged 24 exempt pre-HR1 (2025-05)
• Former foster youth aged 25 NOT exempt pre-HR1 (2025-05)
• Former foster youth aged 24 NOT exempt post-HR1 (2025-08)

IHCIA Indian coverage (exempt post-HR1, no effect pre-HR1) exists as Cases 24–25.

$ uv run policyengine-core test policyengine_us/tests/policy/baseline/gov/usda/snap/eligibility/work_requirements -c policyengine_us
69 passed, 1 warning in 7.91s

$ uv run policyengine-core test policyengine_us/tests/policy/baseline/gov/usda/snap/eligibility -c policyengine_us
155 passed, 1 warning in 36.37s

Note: the new monthly test cases key year-defined inputs by calendar year (e.g. age: {2025: 24}) because non-January monthly periods otherwise trip a period-parsing error in core's input storage.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (f9e58e7) to head (83c934c).
⚠️ Report is 46 commits behind head on main.

Additional details and impacted files

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- Hits            55        41   -14     

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