Medicaid postpartum coverage should run to the end of the month in which the 60-day or 12-month period ends

Medicaid postpartum coverage runs to the end of a calendar month, but the model stops it at a day count.

Law

42 USC 1396a(e)(5): coverage continues "through the end of the month in which the 60-day period (beginning on the last day of her pregnancy) ends." (e)(6) uses the same end point for the income-change protection.
42 USC 1396a(e)(16)(A): the extension runs to "the last day of the month in which the 12-month period (beginning on the last day of her pregnancy) ends."
SHO 21-007, page 3: "The 12-month postpartum period begins on the last day of a beneficiary's pregnancy and extends through the end of the month in which the 12-month period ends." Its examples on page 21 work this out: a pregnancy that ends on August 16, 2022 has its 60-day period end on October 31, 2022 and its 12-month period end on August 31, 2023.

count_days_postpartum (Person, year) is an undated count of days elapsed since the last day of pregnancy, 0 on that day.
- When it is not given, under_60_days_postpartum maps to 0 and under_12_months_postpartum maps to 60.
is_pregnant_for_medicaid_nfc tests count_days_postpartum < postpartum_coverage[state], where the limit is 60 or 365.
The rule predates PR 10241, which notes it in the parameter's NOTE.

The count rule and the month-end rule diverge in two ways:

Count taken on January 1.
- < 60 matches (e)(5) exactly.
- < 365 misses only a count of 365 when the preceding 365 days include February 29. A pregnancy that ended on January 2, 2024 has a count of 365 on January 1, 2025, so the model says not covered. In law, the 12-month period ends January 1, 2025, and coverage runs through January 31, 2025.
Count taken on a later date. The law covers up to 30 or 31 more days than the count rule, because coverage runs to the end of the month in which the period ends.

Changing < to <= does not fix this. With a January 1 count it would wrongly cover a count of 60, whose 60-day period ended on December 31.

Take a dated input: the pregnancy end date or month.
Compute a monthly coverage end: the last day of the month that contains end + 59 days (the 60-day period), or end + 12 months - 1 day (the 12-month period).
Evaluate coverage by month, so that annual results stop depending on the January 1 read.
Keep the postpartum_coverage election dates from PR 10241 as the switch between the two periods.