class_docstr: A MockSessionManager used for runtime tests.
  
  This is done so that our runtime tests don't rely on a specific SessionManager
  implementation.
code: def get_session_info(self, session_id: str) -> Optional[SessionInfo]:
      return self._session_info_by_id.get(session_id, None)
  
imports: import asyncio
  from typing import Callable, Dict, List, Optional
  from unittest import IsolatedAsyncioTestCase, mock
  from streamlit.proto.ForwardMsg_pb2 import ForwardMsg
  from streamlit.runtime import Runtime, RuntimeConfig, RuntimeState
  from streamlit.runtime.app_session import AppSession
  from streamlit.runtime.caching.storage.dummy_cache_storage import MemoryCacheStorageManager
  from streamlit.runtime.memory_media_file_storage import MemoryMediaFileStorage
  from streamlit.runtime.memory_uploaded_file_manager import MemoryUploadedFileManager
  from streamlit.runtime.script_data import ScriptData
  from streamlit.runtime.scriptrunner.script_cache import ScriptCache
  from streamlit.runtime.session_manager import SessionClient, SessionInfo, SessionManager, SessionStorage
  from streamlit.runtime.uploaded_file_manager import UploadedFileManager
label_desc: Use image node_id 4 for calling the MockSessionManager obj's underlying member method code with example usage: obj.get_session_info(session_id) and returns: self