Slice 54: Elmhurst mapper sets extensions_count from len(survey.extensions)

`from_elmhurst_site_notes` hard-coded `extensions_count=0` regardless of
how many extensions the survey lodged. The 6 cohort certs from Slices
47-53 all happened to have 0-2 extensions whose count nothing
load-bearing read, so this latent bug was invisible. Cert 001479
(Summary_001479.pdf, GOV.UK EPB cert 0535-9020-6509-0821-6222) has Main
+ Extension 1 + Extension 2 and is the first cohort cert with a real
API counterpart — accurate `extensions_count` becomes load-bearing the
moment the cross-mapper parity assertion compares API vs Elmhurst
EpcPropertyData side by side.

No SAP-cascade impact (the cascade iterates `sap_building_parts`, not
`extensions_count`) — but a real data-integrity bug surfaced by the
cross-mapper diff. Adds Summary_001479.pdf as a new chain-test fixture
and `_SUMMARY_001479_PDF` constant for follow-up slices that will
land per-bp ages, exposed floors, secondary-heating SAP codes, etc.

All 9 chain tests green; 321 mapper/site-notes/rdsap tests green;
pyright net-zero (35-error baseline preserved on mapper.py).

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

backend/documents_parser/tests/test_summary_pdf_mapper_chain.py

@@ -44,6 +44,7 @@ _SUMMARY_000480_PDF = _FIXTURES / "Summary_000480.pdf"
 _SUMMARY_000487_PDF = _FIXTURES / "Summary_000487.pdf"
 _SUMMARY_000490_PDF = _FIXTURES / "Summary_000490.pdf"
 _SUMMARY_000516_PDF = _FIXTURES / "Summary_000516.pdf"
+_SUMMARY_001479_PDF = _FIXTURES / "Summary_001479.pdf"
 
 
 def _summary_pdf_to_textract_style_pages(pdf_path: Path) -> list[str]:
@@ -249,3 +250,21 @@ def test_summary_000490_full_chain_sap_matches_worksheet_pdf_exactly() -> None:
     # Assert
     worksheet_unrounded_sap = 57.3979
     assert abs(result.sap_score_continuous - worksheet_unrounded_sap) < 1e-4
+
+
+def test_summary_001479_mapper_extensions_count_matches_extension_bps() -> None:
+    # Arrange — cert 0535-9020-6509-0821-6222 (Summary_001479) is the first
+    # cohort cert with an actual GOV.UK API counterpart. Worksheet PDF
+    # lodges Main + Extension 1 + Extension 2 (3 building parts, 2
+    # extensions). Pre-slice the Elmhurst mapper hard-coded
+    # `extensions_count=0` regardless of survey.extensions; this asserts
+    # the count flows through.
+    pages = _summary_pdf_to_textract_style_pages(_SUMMARY_001479_PDF)
+    site_notes = ElmhurstSiteNotesExtractor(pages).extract()
+
+    # Act
+    epc = EpcPropertyDataMapper.from_elmhurst_site_notes(site_notes)
+
+    # Assert
+    assert epc.extensions_count == 2
+    assert len(epc.sap_building_parts) == 3

datatypes/epc/domain/mapper.py

@@ -309,7 +309,7 @@ class EpcPropertyDataMapper:
             has_hot_water_cylinder=survey.water_heating.hot_water_cylinder_present,
             has_fixed_air_conditioning=survey.ventilation.fixed_space_cooling,
             wet_rooms_count=0,
-            extensions_count=0,
+            extensions_count=len(survey.extensions),
             heated_rooms_count=survey.heated_habitable_rooms,
             open_chimneys_count=survey.ventilation.open_chimneys_count,
             habitable_rooms_count=survey.habitable_rooms,