Fix UC childcare work condition for couples

[MaxGhenis]

Summary

• require all adults in a benefit unit to be in work for the UC childcare work condition
• update the couple regression test so one working adult is ineligible and both working adults remain eligible
• add a changelog fragment

EFRS oracle evidence

Using the new Axiom RuleSpec Regulation 32 surface over the local enhanced FRS:

• published policyengine-uk==2.88.42: 5,996 mismatches out of 61,858 benefit units
• this branch installed locally: 0 mismatches out of 61,858 benefit units

The mismatches are all cases where current PE returns true for the childcare work condition while RuleSpec returns false, consistent with the existing formula using any(adults_in_work) instead of requiring all adults in the benefit unit to be in work.

Tests

• uv run policyengine-core test policyengine_uk/tests/policy/baseline/finance/benefit/family/universal_credit/childcare_element/uc_childcare_work_condition.yaml -c policyengine_uk
• uv run policyengine-core test policyengine_uk/tests/policy/baseline/finance/benefit/family/universal_credit/childcare_element -c policyengine_uk
• uv run ruff check policyengine_uk/variables/gov/dwp/universal_credit/childcare_element/uc_childcare_work_condition.py
• uv run ruff format --check policyengine_uk/variables/gov/dwp/universal_credit/childcare_element/uc_childcare_work_condition.py
• git diff --check