[hua7450]

Closes #10366
Closes #10367

What and why

Two formula fixes in the Medicaid medically needy pathway.

Fix	Before	After	Source
Blind people (#10366)	is_in_medicaid_medically_needy_category checked is_ssi_disabled but never is_blind, so a blind adult under 65 who is not SSI-disabled could not qualify	is_blind counts under the existing disabled flag	42 CFR 435.322 (blind) beside 435.324 (disabled)
Financial unit (#10367)	Income, medical expenses and resources were summed over the tax unit, and the couple limits applied when tax_unit_is_joint	Sums run over the marital unit, and the couple limits apply when the marital unit has two people	42 CFR 435.602(a)(2)(i)

Blind under the disabled flag. The disabled parameter cites KFF's 50-state survey, whose medically needy column covers "seniors and people with disabilities" and counts blindness as a disability. KFF never reports blind coverage separately, so this PR adds no blind parameter.

Marital unit. Non-MAGI Medicaid counts only the income and resources of a spouse, or of a parent for a child. ssi_countable_income already carries a child's deemed parental income, an ineligible spouse's deemed income, and each half of an eligible couple's combined income. The optional aged, blind and disabled pathway already sums over the marital unit in the same way (is_optional_senior_or_disabled_income_eligible, medicaid_optional_senior_or_disabled_income_limit).

The couple flag keeps its name, is_joint, so that the income-limit lines stay untouched. The open work on #10357 and #10358 rewrites those lines, and git merge-file merges the two versions cleanly in either order.

Examples

Massachusetts, 2026: $522 / $650 a month and $2,000 / $3,000 in resources.

Household	main	This PR
Blind adult, age 40, not SSI-disabled	Not in a medically needy category	In the disabled group
Aged filer (70): $17,760 countable income, $13,000 medical expenses, $500 resources. Claims their 92-year-old parent: $3,760 countable pension, $1,800 resources	8,520 > 6,264 and 2,300 > 2,000: ineligible	4,760 ≤ 6,264 and 500 ≤ 2,000: eligible
Aged married couple filing separately, $13,080 countable income and $8,000 medical expenses each	Each passes alone: 5,080 ≤ 6,264	10,160 > 7,800 (couple limit): ineligible

Microsimulation effect

• Blind people under 65 who are not SSI-disabled can now qualify as medically needy in the 32 states with the disabled group on, if they fall in no other pathway and meet the limits after medical expenses.
• Aged, blind and disabled people who share a tax unit with relatives other than a spouse, such as parents claimed as dependents, are now tested on their own income and resources against the individual limits. Spouses who file separately are tested together against the couple limits.
• The population effect has not been measured.

Relation to #10240

A local review of #10240 flagged the tax-unit sum (once that PR counts noncitizens' income, an aged dependent's pension removes the filer's eligibility). This PR fixes it, so #10240 need not. Both PRs append cases to is_medically_needy_for_medicaid.yaml as Cases 3 and 4, so whichever merges second renumbers its cases.

Limitations

• When a caller omits marital units, Core puts every household member into one marital unit (Preserve supplied marital units and infer missing explicit couples #10000 addresses this), so any two-person household gets the couple limits. The optional aged, blind and disabled pathway has the same behavior.
• is_joint uses the couple limits whenever the person is married and living with their spouse, whether or not the spouse is aged, blind or disabled. This matches the optional pathway.

Tests

• is_in_medicaid_medically_needy_category.yaml: a blind adult in Massachusetts (in the group) and in Texas, which has no disabled group (not in a group).
• is_medically_needy_for_medicaid.yaml: the filer with a dependent parent, and the couple filing separately (both examples above).
• A static scan of the YAML suite found no existing case whose expected value changes: no multi-person case with an aged, blind or disabled member has a medically needy result that differs between the tax and marital units, and the existing blind cases already qualify through another pathway or live in states without the group. Partner contract tests (tests/policy/baseline/partners/**) are unaffected; their multi-person aged and disabled cases are in Texas, which has no medically needy aged or disabled group.

Test plan

• make format (ruff)
• Changelog fragment in changelog.d/
• CI passes (tests were not run locally)

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