Scale NJ CCAP copay by children actually in care

[MaxGhenis]

Summary

Follow-up to #7811. The New Jersey CCAP copay schedule (CC-236) scales the family co-payment by the number of children actually receiving subsidized care, applying a first-child rate for one child in care and a first + second rate for two in care. The initial implementation counted all age- and immigration-eligible children, so a family with two eligible children but only one in subsidized care would be charged at the two-child rate instead of the one-child rate.

The fix gates the per-child count on an "in care" signal (childcare_hours_per_week > 0, evaluated at the year), following the existing pattern in va_ccsp_copay.py and sc_ccap_copay.py. The CPS-exemption check still references is_eligible_child, so the protective-services waiver is unchanged.

Regulatory authority

• N.J.A.C. 10:15-9.1 -- Co-payment requirements (per-child rates apply to children receiving care).
• CC-236 Copayment Schedule -- first-child / second-child rates.
• See Implement New Jersey CCAP #7811 for the full program description.

Regression tests

Added two cases to nj_ccap_copay.yaml:

• Case 11: two eligible children, only one FT in care, income in the 101-200% FPL tier -- now returns the 1-child FT rate (2%) -> $51.67/mo, not the 2-child FT rate (3%) -> $77.50/mo.
• Case 12: two eligible children, only one PT in care -- now returns the 1-child PT rate (1%) -> $33.33/mo.

Both tests fail on main and pass after this change. All 126 NJ CCAP tests and all 450 NJ-state tests pass.

Test plan

• New regression cases fail without the fix
• New regression cases pass with the fix
• All NJ CCAP tests pass (126/126)
• All NJ state tests pass (450/450)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (05d5d97) to head (c9039bd).
⚠️ Report is 7 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##              main     #8068    +/-   ##
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  Coverage   100.00%   100.00%            
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  Files           18         1    -17     
  Lines          353        29   -324     
  Branches         4         0     -4     
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- Hits           353        29   -324     

Flag	Coverage Δ
unittests	100.00% <100.00%> (ø)

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