class_docstr: A simple SessionStorage implementation used for testing.
  
  Essentially just a thin wrapper around a dict. This class exists so that we don't
  accidentally have our WebsocketSessionManager tests rely on a real SessionStorage
  implementation.
code: def save(self, session_info):
      self._cache[session_info.session.id] = session_info
  
imports: import unittest
  from unittest.mock import MagicMock, patch
  import pytest
  from streamlit.runtime.script_data import ScriptData
  from streamlit.runtime.session_manager import SessionStorage
  from streamlit.runtime.websocket_session_manager import WebsocketSessionManager
label_desc: Use image node_id 3 for calling the MockSessionStorage obj's underlying member method code with example usage: obj.save(session_info) without return types