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 get(self, session_id):
      return self._cache.get(session_id, None)
  
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 2 for calling the MockSessionStorage obj's underlying member method code with example usage: obj.get(session_id) and returns: self