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

crates/servers/storage-server/src/adapters/mock_fs.rs

@@ -116,4 +116,6 @@ impl FileSystem for MockFileSystem {
     fn file_size(&mut self, handle: FileHandle) -> Option<usize> {
         self.files[handle.raw() as usize].as_ref().map(|e| e.size)
     }
+
+    fn mkdir(&mut self, _path: &[u8]) -> bool { true }
 }