Use pension_contributions (employee + personal) and update formula (#1410)

- Changed from employee_pension_contributions to pension_contributions
  which includes both employee and personal pension contributions
- Updated formula to actually subtract pension contributions
- Aligns with HBAI methodology

Fixes #1411

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Max Ghenis <mghenis@gmail.com>
Co-authored-by: Claude <noreply@anthropic.com>

Author: 3 people

policyengine_uk/variables/household/income/household_net_income.py

@@ -16,11 +16,14 @@ class household_net_income(Variable):
     ]
     subtracts = [
         "household_tax",
-        "employee_pension_contributions",
+        "pension_contributions",
     ]
 
     def formula(household, period, parameters):
         market_income = household("household_market_income", period)
         benefits = household("household_benefits", period)
         tax = household("household_tax", period)
-        return np.round(market_income + benefits - tax)
+        pension_contributions = add(
+            household, period, ["pension_contributions"]
+        )
+        return np.round(market_income + benefits - tax - pension_contributions)