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

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

@@ -68,6 +68,13 @@ fn list_dir() {
     assert_eq!(resp, StorageResponse::DirEntries { count: 2 });
 }
 
+#[test]
+fn mkdir_returns_ok() {
+    let mut server = test_server();
+    let resp = server.handle(StorageRequest::Mkdir { path: path(b"subdir") });
+    assert_eq!(resp, StorageResponse::Ok);
+}
+
 #[test]
 fn close_succeeds() {
     let mut server = test_server();