Implement deferred Housing Benefit qualifying-childcare conditions #2236

Summary

Following the user's scope decision for #2229 and its dependent #2234, defer the detailed Housing Benefit qualifying-childcare rules. Use the existing childcare_expenses input for the current correction; do not introduce housing_benefit_qualifying_childcare_charges or require callers/data producers to pre-calculate legal eligibility.

The current correction retains the existing annual age-eligible child count and applies the existing family cap and the new remaining-earnings/tax-credit limit. It does not establish that every supplied expense meets the legal qualifying-care conditions. This issue must remain open after those PRs merge.

Deferred rules and government sources

• Provider categories and charge exclusions: registration/approval and other permitted providers, compulsory education, payments between partners, and care by relatives in the child's home. GB working-age regulation 28(7)–(8); pension-age regulation 31; NI working-age regulation 25 and pension-age regulation 29.
• Ordinary work conditions and exceptions: the working lone parent/couple rules; a nonworking partner's incapacity, hospitalisation or imprisonment; sickness continuation, work immediately before the relevant absence, leave/pay entitlement and applicable time limits. See GB working-age regulation 28(1)–(4), (11)–(15), GB pension-age regulation 31, and the NI equivalents above. An annual amount of statutory sick or maternity pay does not establish all these facts.
• Child-specific paid-care attribution: the statutory cap depends on how many children receive qualifying paid care, not merely how many age-eligible children live in the family. The retained aggregate-spending/age-count approximation can apply the multiple-child cap when only one child receives paid care. GB working-age regulation 27(3), pension-age regulation 30(3), NI working-age regulation 24(3) and pension-age regulation 28(3).
• Exact child-age timing and disability histories: the existing annual age thresholds approximate the September cut-off and do not establish every certification or suspended-payment exception. GB regulation 28(6), (9), (13), with the corresponding pension-age/NI provisions above.

Future implementation plan

1. Inventory and reuse existing spending, age, family, work, benefit and factual leave/provider inputs. Check each proposed reuse against HB's own rule; a UC or Tax-Free Childcare eligibility flag is not automatically equivalent.
2. Identify the smallest set of genuinely missing facts and propose any additional inputs and data mappings for approval before implementation. Do not add an externally pre-assessed qualifying amount.
3. Calculate qualifying spending from raw expenses and those facts, preserving compatibility with existing adult- and child-recorded spending. Distinguish qualifying and nonqualifying costs when a family uses multiple kinds of care.
4. Add targeted source-backed cases for providers/exclusions, work and each approved exception, paid-care child counts, and relevant time boundaries. Document unsupported histories rather than silently assuming them.

The regulations allow average weekly charges to be estimated over an appropriate period of up to a year: GB regulation 28(10) and pension-age regulation 31(10). A transaction-by-transaction ledger is not required by this estimation rule.

Related implementation tracking issue: #2228. Accommodation timing: #2233/#2234. Fixed childcare-cap values remain owned by #1925; do not duplicate them in this follow-up.