fix: harden task3 qdrant compatibility boundaries

harness/tht/adapters/factory.py

@@ -32,12 +32,20 @@ def build_vector_store(cfg: Config, *, require_write: bool = False) -> VectorSto
 
     match resource.type:
         case "qdrant":
+            expected_dimension = cfg.embeddings.dim if cfg.embeddings is not None else None
+            if resource.collection_lifecycle == "require_existing" and (
+                expected_dimension is None or expected_dimension <= 0
+            ):
+                raise ConfigError(
+                    "require_existing Qdrant vectors require an explicit positive embedding dimension"
+                )
             return QdrantVectorStore(
                 base_url=resource.base_url,
                 collection=resource.collection,
                 workspace_id=cfg._workspace_id,
                 workspace_revision=cfg._workspace_revision,
-                expected_dimension=cfg.embeddings.dim if cfg.embeddings is not None else None,
+                expected_dimension=expected_dimension,
+                expected_distance="Cosine",
                 collection_lifecycle=resource.collection_lifecycle,
             )
         case other:  # pragma: no cover - Pydantic's discriminator rejects this first.