Illinois aged, blind and disabled Medicaid: give each applying spouse the $25 exemption and leave out contributions, as for the Medicare Savings Programs

Problem

Illinois's $25 income exemption is one rule, PM 15-04-03-a, applied to two Medicaid pathways. PolicyEngine treats the two differently.

Medicare Savings Programs. Apply state Medicare Savings Program asset limits and income exclusions (WA, CA, MN, IL, MS) #10360 gives each applying spouse their own $25, capped at that spouse's own income other than SSI and contributions (state_general_exclusion_per_spouse, state_general_exclusion_excludes_contributions).
Optional aged, blind and disabled pathway. policyengine_us/parameters/gov/hhs/medicaid/eligibility/categories/senior_or_disabled/income/disregard/couple.yaml gives Illinois one $25 per couple. medicaid_optional_senior_or_disabled_countable_income subtracts it once from the couple's combined income, and it also applies the $25 to contributions.

IL DHS PM 15-04-03-a, AABD Income Disregard (Community Cases):
- "The first $25 of earned or unearned income, other than SSI income and contributions, is exempt when determining eligibility for medical."
- "If 2 persons in the standard are applying for or receiving AABD Medical, one $25 income exemption is allowed if only one person has income other than SSI or contributions. Two $25 income exemptions are allowed if both persons have income other than SSI or contributions."
- "A contribution is income received from a person outside the medical unit (i.e., child support, alimony)."
89 Ill. Adm. Code 120.335(a)(1): "the first $25.00 of a client's earned or unearned income other than SSI income, or contributions from a spouse or other individual, is exempt from consideration in determining eligibility."

A couple who both apply and both have income other than SSI and contributions is allowed $50 a month under the manual. The model allows $25, so their countable income is $25 a month too high.
A person whose only income is alimony, child support or outside cash help gets the $25 in the model. Under PM 15-04-03-a they get no $25 exemption.

For the optional aged, blind and disabled pathway in Illinois, give each applying spouse a $25 exemption capped at that spouse's own income other than SSI and contributions, as msp_countable_income does from Apply state Medicare Savings Program asset limits and income exclusions (WA, CA, MN, IL, MS) #10360.
Reuse contribution_sources and the per-spouse design, or move them to a place both pathways can read, rather than copying them.
Add YAML cases: a couple who both have Social Security; a couple where one spouse has only alimony; and a single person whose only income is child support.