Medicaid CE: model full AI/AN and IHS exclusion

[daphnehanse11]

Parent issue: #7751

PR #8257 adds the currently available Indian Health Service coverage proxy, has_indian_health_service_coverage_at_interview, to the Medicaid community engagement exclusion rollup. That still does not fully model the American Indian and Alaska Native exclusion described in the guidance.

Scope:

• Confirm whether the modeled exclusion should be AI/AN status, IHS eligibility, IHS receipt, tribal health service eligibility, or some combination.
• Decide whether PolicyEngine needs a new person-level input for AI/AN status or IHS eligibility.
• Avoid using is_on_tribal_land as a person-level proxy unless the limitation is explicit.
• Extend Medicaid CE tests to cover the chosen input and keep the existing IHS coverage case.

Out of scope:

• State-specific outreach or verification workflow.
• County or geography-based approximations unless explicitly documented as a proxy.

Sources:

• https://www.congress.gov/bill/119th-congress/house-bill/1/text
• https://www.medicaid.gov/federal-policy-guidance/downloads/cib12082025.pdf