restore transaction_type; keep tenure dropped (v2.0.0 stands)

User reverted the transaction_type drop after noting that it doesn't help
detect full-SAP assessments (that's `assessment_type` on the bulk-register
record, filtered out at build_features.py:37).

tenure removal stays; v2.0.0 still MAJOR (a column was removed).

packages/domain/src/domain/ml/tests/test_transform.py

@@ -257,6 +257,7 @@ _NULLABLE_CATEGORICAL_FEATURES: tuple[str, ...] = (
 
 _NON_NULLABLE_CATEGORICAL_FEATURES: tuple[str, ...] = (
     "dwelling_type",
+    "transaction_type",
 )
 
 
@@ -287,6 +288,7 @@ def test_to_row_extracts_categorical_features() -> None:
     epc = make_minimal_sap10_epc(
         energy_rating_current=82,
         dwelling_type="End-terrace house",
+        transaction_type="8",
         property_type="0",
         built_form="2",
         region_code="6",
@@ -300,7 +302,7 @@ def test_to_row_extracts_categorical_features() -> None:
     # Assert
     assert row["dwelling_type"] == "End-terrace house"
     assert "tenure" not in row
-    assert "transaction_type" not in row
+    assert row["transaction_type"] == "8"
     assert row["property_type"] == "0"
     assert row["built_form"] == "2"
     assert row["region_code"] == "6"

packages/domain/src/domain/ml/transform.py

@@ -133,6 +133,12 @@ _FEATURE_COLUMNS: dict[str, ColumnSpec] = {
         categorical=True,
         description="Free-form SAP dwelling-type description, e.g. 'Mid-terrace house'.",
     ),
+    "transaction_type": ColumnSpec(
+        dtype=str,
+        nullable=False,
+        categorical=True,
+        description="SAP transaction type code, stringified int.",
+    ),
     "property_type": ColumnSpec(
         dtype=str,
         nullable=True,
@@ -999,6 +1005,7 @@ class EpcMlTransform:
             "percent_draughtproofed": epc.percent_draughtproofed,
             # Features — categoricals (raw strings; cast at parquet write time)
             "dwelling_type": epc.dwelling_type,
+            "transaction_type": epc.transaction_type,
             "property_type": epc.property_type,
             "built_form": epc.built_form,
             "region_code": epc.region_code,