Apply Mississippi TANF's $90 work expense and dependent care deductions and $10 minimum payment

[PavelMakarchuk]

Summary

ms_tanf_countable_income added gross earned and unearned income with no deductions, and ms_tanf paid any positive amount.

The MDHS TANF Policy Manual (July 2019), Volume III says:

• Pages 6025-6026, "Budgeting Net Monthly Earned Income": for families that pass the gross income test, disregard from the monthly earned income of each individual in the budget (a) a "Standard deduction of $90 for work expenses", then (b) the actual cost of care for each dependent child or incapacitated adult in the budget, with no cap. Page 4104 lists the same disregards.
• Page 6015, "Minimum Benefit", and Chapter 15, Table II: no benefit is issued when the monthly amount is less than $10.

Changes:

• New parameter gov.states.ms.dhs.tanf.income.deductions.work_expense ($90 a month, per earner).
• New parameter gov.states.ms.dhs.tanf.minimum_payment ($10 a month).
• New variable ms_tanf_countable_earned_income: the sum of each earner's max(gross earnings − $90, 0), minus childcare_expenses and care_expenses, floored at zero. Dependent care reduces only earned income.
• ms_tanf_countable_income now adds ms_tanf_countable_earned_income and tanf_gross_unearned_income.
• ms_tanf returns 0 when the benefit is under $10.

The gross income test (185% of the need standard) still uses gross income, as the manual requires. These cases are still not modeled: the 6-month and 3-month total disregards, the student earnings disregard, and the dependent care exceptions (care by the payee's child under 21, or care paid by a third party).

Tests

• New ms_tanf_countable_earned_income.yaml (6 cases): no earnings (control), $200 a month gives 200 − 90 = $110, $50 a month of child care gives $60, two earners at $500 and $300 with $100 of care give 410 + 210 − 100 = $520, earnings under $90 give $0, and care above earnings does not reduce unearned income (countable income $50).
• ms_tanf.yaml: a $5 benefit is not issued (the existing case that expected $5 now expects $0), and a $10 benefit is issued.
• integration.yaml: the issue's case is a parent of two with $2,400 a year of earnings. ms_tanf = 12 × (260 − 110) = $1,800 (main: $720). With $600 a year of child care, ms_tanf = 12 × (260 − 60) = $2,400.

Against main's code, all 9 new or changed earnings and minimum assertions fail. With this PR, policyengine_us/tests/policy/baseline/gov/states/ms/dhs/tanf passes (75 tests), as do gov/hhs/tanf/cash (36) and the parameter, registry and code-health checks (819).

Fixes #10209

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