Normalize RdSAP hot-water demand (WWHRS rooms) on the degraded 17.0/19.0 paths

PRD #1385, shower/WWHRS family. The 17.0 and 19.0 API mappers left
sap_heating.number_baths / mixer_shower_count as None, so the water-heating
demand (cert_to_inputs) fell back to defaults instead of the lodged bath/shower
room counts. Derive them from schema.sap_heating.instantaneous_wwhrs, mirroring
the 17.1/18.0/20.0 paths:

  number_baths       = rooms_with_bath_and_or_shower + rooms_with_bath_and_mixer_shower
  mixer_shower_count = rooms_with_mixer_shower_no_bath + rooms_with_bath_and_mixer_shower

Guarded on instantaneous_wwhrs is not None (Optional in the schema), like 17.1.
Counts, same meaning across the reduced-field family.

Verification: new test 2 passed; test_mapper_corpus 6002 passed; SAP-accuracy /
RealCertExpectation regressions pass; pyright unchanged (39 -> 39, pre-existing).

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

datatypes/epc/domain/mapper.py

@@ -620,6 +620,22 @@ class EpcPropertyDataMapper:
             ),
             sap_heating=SapHeating(
                 instantaneous_wwhrs=InstantaneousWwhrs(),
+                # ADR-0028: derive HW-demand counts from the lodged bath/shower
+                # ROOM counts, mirroring the 17.1/18.0/20.0 paths. These were
+                # dropped (left None), so the water-heating demand fell back to
+                # defaults instead of the lodged bath/mixer-shower counts.
+                number_baths=(
+                    schema.sap_heating.instantaneous_wwhrs.rooms_with_bath_and_or_shower
+                    + schema.sap_heating.instantaneous_wwhrs.rooms_with_bath_and_mixer_shower
+                    if schema.sap_heating.instantaneous_wwhrs is not None
+                    else None
+                ),
+                mixer_shower_count=(
+                    schema.sap_heating.instantaneous_wwhrs.rooms_with_mixer_shower_no_bath
+                    + schema.sap_heating.instantaneous_wwhrs.rooms_with_bath_and_mixer_shower
+                    if schema.sap_heating.instantaneous_wwhrs is not None
+                    else None
+                ),
                 main_heating_details=[
                     MainHeatingDetail(
                         has_fghrs=d.has_fghrs == "Y",
@@ -1602,6 +1618,22 @@ class EpcPropertyDataMapper:
             ),
             sap_heating=SapHeating(
                 instantaneous_wwhrs=InstantaneousWwhrs(),
+                # ADR-0028: derive HW-demand counts from the lodged bath/shower
+                # ROOM counts, mirroring the 17.1/18.0/20.0 paths. These were
+                # dropped (left None), so the water-heating demand fell back to
+                # defaults instead of the lodged bath/mixer-shower counts.
+                number_baths=(
+                    schema.sap_heating.instantaneous_wwhrs.rooms_with_bath_and_or_shower
+                    + schema.sap_heating.instantaneous_wwhrs.rooms_with_bath_and_mixer_shower
+                    if schema.sap_heating.instantaneous_wwhrs is not None
+                    else None
+                ),
+                mixer_shower_count=(
+                    schema.sap_heating.instantaneous_wwhrs.rooms_with_mixer_shower_no_bath
+                    + schema.sap_heating.instantaneous_wwhrs.rooms_with_bath_and_mixer_shower
+                    if schema.sap_heating.instantaneous_wwhrs is not None
+                    else None
+                ),
                 main_heating_details=[
                     MainHeatingDetail(
                         has_fghrs=d.has_fghrs == "Y",

tests/datatypes/epc/domain/test_mapper_wwhrs_parity.py

@@ -0,0 +1,47 @@
+"""Regression: the degraded RdSAP paths (17.0, 19.0) must derive the hot-water
+bath/shower demand counts from the lodged WWHRS room counts, like the
+reference-complete 17.1/18.0/20.0 paths.
+
+PRD #1385 (mapper normalization), shower/WWHRS family. Both paths left
+`sap_heating.number_baths` / `mixer_shower_count` as None, so the water-heating
+demand (`cert_to_inputs`) fell back to defaults. The reference derivation:
+  number_baths       = rooms_with_bath_and_or_shower + rooms_with_bath_and_mixer_shower
+  mixer_shower_count = rooms_with_mixer_shower_no_bath + rooms_with_bath_and_mixer_shower
+"""
+
+from __future__ import annotations
+
+import json
+from pathlib import Path
+from typing import Any
+
+from datatypes.epc.domain.mapper import EpcPropertyDataMapper
+
+# 17.0: rooms bath_or_shower=4, mixer_no_bath=0, bath_and_mixer=4
+#       → number_baths 8, mixer_shower_count 4.
+_FIXTURE_17_0 = Path("tests/fixtures/epc_prediction/W104HL/cert-f96b9b6f471b.json")
+# 19.0: rooms bath_or_shower=2, mixer_no_bath=1, bath_and_mixer=1
+#       → number_baths 3, mixer_shower_count 2.
+_FIXTURE_19_0 = Path("tests/fixtures/epc_prediction/LE113PX/cert-ba628e4f9cbf.json")
+
+
+def _load(path: Path, schema_type: str) -> dict[str, Any]:
+    data: dict[str, Any] = json.loads(path.read_text())
+    assert data["schema_type"] == schema_type
+    return data
+
+
+def test_17_0_derives_hot_water_demand_from_wwhrs_rooms() -> None:
+    epc = EpcPropertyDataMapper.from_api_response(
+        _load(_FIXTURE_17_0, "RdSAP-Schema-17.0")
+    )
+    assert epc.sap_heating.number_baths == 8
+    assert epc.sap_heating.mixer_shower_count == 4
+
+
+def test_19_0_derives_hot_water_demand_from_wwhrs_rooms() -> None:
+    epc = EpcPropertyDataMapper.from_api_response(
+        _load(_FIXTURE_19_0, "RdSAP-Schema-19.0")
+    )
+    assert epc.sap_heating.number_baths == 3
+    assert epc.sap_heating.mixer_shower_count == 2