Pay couple Carer Premium when a benunit has 3+ carers (#1623)

* Pay couple Carer Premium when a benunit has 3+ carers

The select list used carers == 2, so any benefit unit with 3 or more
carers silently fell through to the numpy default of 0. A benefit unit
with e.g. two adult carers plus a young-carer child meeting
is_carer_for_benefits is the realistic way this arises. The Carer
Premium regulations don't define a rate for >2 carers, so clamping to
the couple rate is the correct behaviour.

Rewrote the select as [carers >= 2, carers == 1] → [couple, single],
which also drops the now-redundant carers == 0 branch (select's default
is 0 already).

Fixes #420.

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

* Add regression test for #420 (three-carer benefit unit)

Covers the carers >= 2 branch of carer_premium with an explicit
num_carers=3 case that would have returned 0 under the previous
select. Also adds num_carers=0 and num_carers=2 cases so the full
branch table is exercised.

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

---------

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

Author: vahid-ahmadi

changelog.d/420.md

@@ -0,0 +1 @@
+- Pay the two-carer Carer Premium rate when a benefit unit has three or more carers. The previous `carers == 2` condition silently returned £0 for the (rare) case of three or more carers in the same benefit unit; the Carer Premium regulations don't define a higher rate, so the couple rate is the correct ceiling.

policyengine_uk/tests/policy/baseline/finance/benefit/care.yaml

@@ -7,3 +7,27 @@
     is_carer_for_benefits: true
     num_carers: 1
     carer_premium: 37.5 * 52
+
+- name: Two carers in a benefit unit receive the couple rate
+  period: 2020
+  absolute_error_margin: 0
+  input:
+    num_carers: 2
+  output:
+    carer_premium: 37.5 * 52
+
+- name: Three carers in a benefit unit stay at the couple rate (regression for #420)
+  period: 2020
+  absolute_error_margin: 0
+  input:
+    num_carers: 3
+  output:
+    carer_premium: 37.5 * 52
+
+- name: No carers in a benefit unit receive no premium
+  period: 2020
+  absolute_error_margin: 0
+  input:
+    num_carers: 0
+  output:
+    carer_premium: 0

policyengine_uk/variables/household/demographic/carer_premium.py

@@ -13,7 +13,7 @@ def formula(benunit, period, parameters):
         carers = benunit("num_carers", period.this_year)
         CP = parameters(period).gov.dwp.carer_premium
         weekly_premium = select(
-            [carers == 0, carers == 1, carers == 2],
-            [0, CP.single, CP.couple],
+            [carers >= 2, carers == 1],
+            [CP.couple, CP.single],
         )
         return weekly_premium * WEEKS_IN_YEAR