Handle looked-after children in Tax-Free Childcare

[MaxGhenis]

Summary

• Add a default-false is_looked_after_by_local_authority person input, referenced to Childcare Payments (Eligibility) Regulations 2015 regulation 4.
• Add tax_free_childcare_qualifying_child, combining the existing age rule with non-parent and looked-after-child exclusions.
• Use qualifying-child status for Tax-Free Childcare eligibility, per-child payment caps, and receiving-status detection.

Closes #1048.

Notes

• The issue referenced regulation 16, but current legislation.gov.uk has the foster/looked-after-child responsibility rule in regulation 4 and the age qualifying-child rule in regulation 5.
• Baseline behavior is unchanged unless the new looked-after-child input is supplied, except that per-child payment caps now depend on the explicit qualifying-child condition rather than just ~is_parent.

Tests

• uv run --python 3.13 policyengine-core test policyengine_uk/tests/policy/baseline/gov/hmrc/tax_free_childcare -c policyengine_uk
• uv run --python 3.13 ruff check policyengine_uk/variables/household/demographic/is_looked_after_by_local_authority.py policyengine_uk/variables/gov/hmrc/tax_free_childcare/conditions/tax_free_childcare_qualifying_child.py policyengine_uk/variables/gov/hmrc/tax_free_childcare/tax_free_childcare_eligibility.py policyengine_uk/variables/gov/hmrc/tax_free_childcare/tax_free_childcare.py policyengine_uk/variables/gov/hmrc/tax_free_childcare/is_child_receiving_tax_free_childcare.py
• uv run --python 3.13 ruff format --check policyengine_uk/variables/household/demographic/is_looked_after_by_local_authority.py policyengine_uk/variables/gov/hmrc/tax_free_childcare/conditions/tax_free_childcare_qualifying_child.py policyengine_uk/variables/gov/hmrc/tax_free_childcare/tax_free_childcare_eligibility.py policyengine_uk/variables/gov/hmrc/tax_free_childcare/tax_free_childcare.py policyengine_uk/variables/gov/hmrc/tax_free_childcare/is_child_receiving_tax_free_childcare.py