Route a water_heating override row through its overlay mapper 🟥

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

tests/repositories/property/test_landlord_override_overlays.py

@@ -92,6 +92,26 @@ def test_construction_age_band_row_produces_a_building_part_overlay() -> None:
     assert main.construction_age_band == "B"
 
 
+def test_water_heating_row_produces_a_heating_overlay() -> None:
+    # Arrange
+    overrides = ResolvedPropertyOverrides(
+        rows=(
+            ResolvedPropertyOverride(
+                "water_heating", 0, "From main system, mains gas"
+            ),
+        )
+    )
+
+    # Act
+    overlays = overlays_from(overrides)
+
+    # Assert
+    assert len(overlays) == 1
+    assert overlays[0].heating is not None
+    assert overlays[0].heating.water_heating_code == 901
+    assert overlays[0].heating.water_heating_fuel == 26
+
+
 def test_unresolvable_rows_are_skipped() -> None:
     # Arrange — an "Unknown" property type and an unmapped wall material.
     overrides = ResolvedPropertyOverrides(