Harden split background promotion preflight

scripts/run_split_background_promotion_workflow.sh

@@ -110,11 +110,24 @@ samples = payload.get("samples") or payload.get("items") or []
 if not isinstance(samples, list) or not samples:
     raise SystemExit(f"Operator sample manifest has no samples: {manifest_path}")
 
-background_categories = {
-    str(sample.get("background_category") or "")
-    for sample in samples
-    if str(sample.get("sample_role") or "") == "background_candidate"
-}
+background_categories = set()
+for sample in samples:
+    if str(sample.get("sample_role") or "") != "background_candidate":
+        continue
+
+    category = str(sample.get("background_category") or "").strip()
+    if not category:
+        try:
+            reference_feature_count = int(sample.get("reference_feature_count") or 0)
+        except (TypeError, ValueError):
+            reference_feature_count = 0
+        category = (
+            "pure_empty_negative"
+            if reference_feature_count == 0
+            else "sparse_building_context"
+        )
+
+    background_categories.add(category)
 missing = {"pure_empty_negative", "sparse_building_context"} - background_categories
 if missing:
     raise SystemExit(