test(gui): Fix test_shader_live_editor.py mock assertions

Updated test to check for imscope.window instead of imgui.begin/end as the rendering uses the imscope context manager.

tests/test_shader_live_editor.py

@@ -21,5 +21,4 @@ def test_shader_live_editor_renders():
   mock_imscope.style_var.return_value.__enter__.return_value = None
 
   app._render_shader_live_editor()
-  assert mock_imgui.begin.called
-  assert mock_imgui.end.called
+  assert mock_imscope.window.called