Decode more water-heating system/fuel overrides to their codes 🟥

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/domain/epc/test_water_heating_overlay.py

@@ -6,6 +6,8 @@ A water-heating value resolves to the SAP `water_heating_code` (system) and
 
 from __future__ import annotations
 
+import pytest
+
 from domain.epc.property_overlays.water_heating_overlay import (
     water_heating_overlay_for,
 )
@@ -20,3 +22,34 @@ def test_from_main_system_mains_gas_overlays_water_heating() -> None:
     assert simulation.heating is not None
     assert simulation.heating.water_heating_code == 901
     assert simulation.heating.water_heating_fuel == 26
+
+
+@pytest.mark.parametrize(
+    ("water_heating_value", "code", "fuel"),
+    [
+        ("From main system, electricity", 901, 29),
+        ("Electric immersion, electricity", 903, 29),
+    ],
+)
+def test_water_heating_systems_decode_to_their_codes(
+    water_heating_value: str, code: int, fuel: int
+) -> None:
+    # Act
+    simulation = water_heating_overlay_for(water_heating_value, 0)
+
+    # Assert
+    assert simulation is not None
+    assert simulation.heating is not None
+    assert simulation.heating.water_heating_code == code
+    assert simulation.heating.water_heating_fuel == fuel
+
+
+@pytest.mark.parametrize("water_heating_value", ["Unknown", ""])
+def test_unresolvable_water_heating_produces_no_overlay(
+    water_heating_value: str,
+) -> None:
+    # Act
+    simulation = water_heating_overlay_for(water_heating_value, 0)
+
+    # Assert
+    assert simulation is None