Use np.isin for country check in winter_fuel_allowance (#257) (#1650)

One of the last OR-chained equality checks in the codebase. All 6
baseline winter-fuel YAML tests still pass.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: vahid-ahmadi

changelog.d/257-winter-fuel.md

@@ -0,0 +1 @@
+- Use `np.isin` instead of OR-chained equality in `winter_fuel_allowance`.

policyengine_uk/variables/gov/dwp/winter_fuel_allowance.py

@@ -29,7 +29,7 @@ def formula(household, period, parameters):
         taxable_income = household.members("total_income", period)
         is_SP_age = household.members("is_SP_age", period)
         country = household("country", period).decode_to_str()
-        in_england_or_wales = (country == "ENGLAND") | (country == "WALES")
+        in_england_or_wales = np.isin(country, ["ENGLAND", "WALES"])
         meets_income_passport = (
             household.any(
                 is_SP_age