feat(storage): add mkdir to FileSystem trait with stubs

crates/servers/storage-server/src/tests/protocol.rs

@@ -59,6 +59,13 @@ fn all_response_variants() {
     }
 }
 
+#[test]
+fn mkdir_roundtrip() {
+    let path = PackedPath::from_bytes(b"mydir");
+    let req = StorageRequest::Mkdir { path };
+    assert_eq!(StorageRequest::decode(req.encode()), Some(req));
+}
+
 #[test]
 fn invalid_request_returns_none() {
     assert!(StorageRequest::decode([99, 0, 0, 0]).is_none());