Add UC carer element to extended childcare exemptions

[MaxGhenis]

Summary

• Add uc_carer_element to the extended childcare entitlement disability criteria parameter list.
• Update the work-condition formula to evaluate mixed person-level and benefit-unit-level criteria from that list.
• Add a regression test for Carer's Allowance while retaining the UC carer element test.

Closes #1032.

Tests

• uv run --python 3.13 policyengine-core test policyengine_uk/tests/policy/baseline/gov/dfe/extended_childcare_entitlement -c policyengine_uk
• uv run --python 3.13 ruff check policyengine_uk/variables/gov/dfe/extended_childcare_entitlement/conditions/extended_childcare_entitlement_work_condition.py
• uv run --python 3.13 ruff format --check policyengine_uk/variables/gov/dfe/extended_childcare_entitlement/conditions/extended_childcare_entitlement_work_condition.py