fix: harden task3 qdrant compatibility boundaries

harness/tests/test_adapter_factory.py

@@ -67,6 +67,15 @@ def test_factory_selects_dwh_adapter(dwh_type, adapter_type):
     assert isinstance(build_dwh(_config(dwh_type=dwh_type)), adapter_type)
 
 
+def test_factory_rejects_require_existing_without_embedding_dimension():
+    config = _config()
+    config.vectors.collection_lifecycle = "require_existing"
+    config.embeddings = None
+
+    with pytest.raises(ConfigError, match="explicit positive embedding dimension"):
+        build_vector_store(config)
+
+
 def test_factory_selects_qdrant_for_schema_v3_runtime():
     config = _config(dwh_type="postgres_direct")