Test winter heating payments for couples on Income Support, income-based JSA and income-related ESA #2389

The Winter Fuel Payment and Pension Age Winter Heating Payment YAML suites (relevant_benefit_receipt.yaml, pawhp.yaml) test couples on Pension Credit, Universal Credit and income-related ESA, but:

• no case puts a couple on Income Support;
• no case puts an income-based JSA or income-related ESA couple of pensionable age on the one-payment rule (SI 2025/969 reg. 4(2)(a); SI 2024/869 reg. 4(1)(a)(i); SSI 2024/351 reg. 9(d));
• income-based JSA mixed-age couples are tested only for the 2023 week.

Since #2027, is_on_income_support, is_on_income_based_jsa and is_on_income_related_esa are true only for the member of a couple the award is paid to. #2038 handles this through partner_receipt_counts and the couple-level read in is_excluded_relevant_benefit_partner, and the Hypothesis properties cover it statistically. But a deterministic regression is missing for these awards.

Mutation check on main (e01f2f5): these three mutants pass both suites:

• is_on_income_support true for both members of a couple;
• Income Support dropped from the WFP relevant-benefit list;
• Income Support dropped from the PAWHP relevant-benefit list.