Align Tax-Free Childcare UC disqualification

[MaxGhenis]

Summary

• Change the Tax-Free Childcare UC disqualification proxy from final universal_credit to universal_credit_pre_benefit_cap, matching the Act's rule that UC counts as payable even when reduced to nil.
• Update the disqualifying-benefits parameter references from the draft SI to current Childcare Payments Act sections.
• Add a regression test for UC payable before a nil final award.

Closes #1056.

Audit note for #1056

• Universal childcare entitlement has no qualifying-benefit condition to switch.
• Extended childcare uses specified-benefit entitlement/award amounts for the work-condition exemption; Add UC carer element to extended childcare exemptions #1604 moved UC carer element into that list.
• Tax-Free Childcare is claim/award-based rather than pure eligibility-based; this PR tightens UC to the payable-before-nil-reduction concept in section 11.

Tests

• uv run --python 3.13 policyengine-core test policyengine_uk/tests/policy/baseline/gov/hmrc/tax_free_childcare/tax_free_childcare_program_eligible.yaml -c policyengine_uk
• uv run --python 3.13 policyengine-core test policyengine_uk/tests/policy/baseline/gov/hmrc/tax_free_childcare -c policyengine_uk