code: def test_return_type(self, test_plot) -> None:
      r = bes.server_session(test_plot, session_id="fakesession")
      assert isinstance(r, str)
  
imports: from __future__ import annotations
  import pytest
  import bs4
  import bokeh.embed.server
label_desc: Use image node_id 1 for calling the TestServerSession obj's underlying member method code with example usage: obj.test_return_type(test_plot) without return types