Treat qualifying leave as work for Tax-Free Childcare

[MaxGhenis]

Summary

• Add tax_free_childcare_on_qualifying_leave for qualifying leave periods not represented by existing income variables.
• Add tax_free_childcare_treated_as_in_work, treating raw work, qualifying leave, statutory sick pay, maternity allowance, statutory maternity pay, and statutory paternity pay as work for Tax-Free Childcare.
• Route tax_free_childcare_work_condition through the new treated-as-in-work helper for single and couple conditions.

Closes #1044.

Notes

• The issue references regulations 8-10, but current legislation.gov.uk has temporary absence/parenting leave treatment in regulation 12, with the work requirement cross-reference in regulation 14.
• This does not add distinct statutory adoption/shared parental pay income variables; the new qualifying-leave input covers those leave states until those incomes are modeled directly.

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/gov/hmrc/tax_free_childcare/conditions/tax_free_childcare_on_qualifying_leave.py policyengine_uk/variables/gov/hmrc/tax_free_childcare/conditions/tax_free_childcare_treated_as_in_work.py policyengine_uk/variables/gov/hmrc/tax_free_childcare/conditions/tax_free_childcare_work_condition.py
• uv run --python 3.13 ruff format --check policyengine_uk/variables/gov/hmrc/tax_free_childcare/conditions/tax_free_childcare_on_qualifying_leave.py policyengine_uk/variables/gov/hmrc/tax_free_childcare/conditions/tax_free_childcare_treated_as_in_work.py policyengine_uk/variables/gov/hmrc/tax_free_childcare/conditions/tax_free_childcare_work_condition.py