S0380.221: default a missing API post_town so the cert stays mappable

A 2026-register cert (4519-9056-4002-0222-4802) omits the top-level
post_town entirely (its town sits only in address_line_3 "BARNSTAPLE").
RdSapSchema21_0_x declares post_town as a required no-default field, so
from_dict raised "missing required field 'post_town'" and blocked the
whole cert from computing.

post_town is address metadata the SAP cascade never reads (no consumer
in domain/sap10_calculator/), so default an absent post_town to "" in a
from_api_response pre-processor (mirroring _normalize_shower_outlets) —
inert for the calculation, keeps the cert mappable. The schema dataclass
can't simply give post_town a default: it is a plain (non-kw_only)
dataclass with 57 required fields after post_town, so a mid-list default
would break field ordering.

Validated: cert 4519 now maps (post_town="") and computes SAP cont 74.68
vs lodged 75. §4 suite 2392 passed; mapper.py pyright unchanged at 32;
new tests suppress reportPrivateUsage (net-zero).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

datatypes/epc/domain/mapper.py

@@ -1911,6 +1911,7 @@ class EpcPropertyDataMapper:
         """
 
         data = _normalize_shower_outlets(data)
+        data = _default_missing_post_town(data)
         schema = data.get("schema_type", "")
         if schema == "RdSAP-Schema-21.0.1":
             from datatypes.epc.schema.rdsap_schema_21_0_1 import RdSapSchema21_0_1
@@ -2081,6 +2082,26 @@ def _normalize_shower_outlets(data: Dict[str, Any]) -> Dict[str, Any]:
     return {**data, "sap_heating": new_sap_heating}
 
 
+def _default_missing_post_town(data: Dict[str, Any]) -> Dict[str, Any]:
+    """Default an absent top-level `post_town` to "" before `from_dict`.
+
+    `RdSapSchema21_0_x.post_town` is a required (no-default) field, so a
+    real-API cert that omits it (observed on a 2026-register cert whose
+    town sits only in `address_line_3`) makes `from_dict` raise
+    "missing required field 'post_town'", blocking the whole cert.
+    `post_town` is address metadata that the SAP cascade never reads, so
+    defaulting it to "" is inert for the calculation while keeping the
+    cert mappable. The schema dataclass can't simply give the field a
+    default — it is a plain (non-kw_only) dataclass with 57 required
+    fields after `post_town`, so a mid-list default would break field
+    ordering; pre-processing here mirrors `_normalize_shower_outlets`.
+
+    Mutates a shallow copy so the caller's dict is untouched."""
+    if "post_town" in data:
+        return data
+    return {**data, "post_town": ""}
+
+
 def _count_shower_outlets_by_type(
     schema_shower_outlets: Any, target_type: int,
 ) -> Optional[int]:

datatypes/epc/domain/tests/test_from_rdsap_schema.py

@@ -849,3 +849,33 @@ class TestApiFloorConstructionCode:
 
         # Assert — no raise; None defers to the cascade's Table 19 default.
         assert result is None
+
+
+class TestDefaultMissingPostTown:
+    """`_default_missing_post_town` keeps a cert mappable when the
+    register omits the required `post_town` field (observed on a 2026
+    cert whose town sits only in address_line_3). post_town is address
+    metadata the SAP cascade never reads, so defaulting it to "" before
+    `from_dict` is inert for the calculation."""
+
+    def test_absent_post_town_is_defaulted_to_empty_string(self) -> None:
+        # Arrange
+        from datatypes.epc.domain.mapper import _default_missing_post_town  # pyright: ignore[reportPrivateUsage]
+        doc: dict[str, object] = {"postcode": "EX31 2LE"}
+
+        # Act
+        result = _default_missing_post_town(doc)
+
+        # Assert
+        assert result["post_town"] == ""
+
+    def test_present_post_town_is_untouched(self) -> None:
+        # Arrange — regression guard: a lodged town passes through.
+        from datatypes.epc.domain.mapper import _default_missing_post_town  # pyright: ignore[reportPrivateUsage]
+        doc: dict[str, object] = {"post_town": "BARNSTAPLE"}
+
+        # Act
+        result = _default_missing_post_town(doc)
+
+        # Assert
+        assert result["post_town"] == "BARNSTAPLE"