fix: make vector ingestion canonical and atomic

backend/tests/test_geojson_dataset_service.py

@@ -117,6 +117,24 @@ def test_parse_geojson_payload_returns_vector_metadata() -> None:
     assert metadata["approximate_area_m2"] >= 0.0
 
 
+def test_parse_geojson_payload_reports_z_dimension_for_canonical_2d_storage() -> None:
+    metadata = geojson_service.parse_geojson_payload(
+        {
+            "type": "FeatureCollection",
+            "features": [
+                {
+                    "type": "Feature",
+                    "geometry": {"type": "Point", "coordinates": [5.08, 51.18, 0.0]},
+                    "properties": {},
+                }
+            ],
+        }
+    )
+
+    assert metadata["z_dimension_feature_count"] == 1
+    assert metadata["canonical_storage_dimension"] == "2D"
+
+
 def test_parse_geojson_payload_rejects_invalid_geometry() -> None:
     payload = {
         "type": "FeatureCollection",