Tighten child and qualifying young person benefit rules

[MaxGhenis]

Summary

• tighten the generic qualifying young person rule to require non-advanced education or approved training and exclude people receiving adult benefits in their own right
• add program-specific child/QYP predicates for UC, CTC, Child Benefit, and Pension Credit
• use UC-specific child ordering for UC child elements and update tests for the legal default behavior

Tests

• uv run pytest policyengine_uk/tests/test_child_qyp_rules.py -q
• uvx ruff format --check <touched Python files>
• uv run policyengine-core test policyengine_uk/tests/policy -c policyengine_uk