Nest TODO State in session data (#4361)

Co-authored-by: Alex Hancock <alexhancock@block.xyz>

crates/goose/tests/test_support.rs

@@ -411,6 +411,6 @@ pub fn create_test_session_metadata(message_count: usize, working_dir: &str) ->
         accumulated_total_tokens: Some(100),
         accumulated_input_tokens: Some(50),
         accumulated_output_tokens: Some(50),
-        todo_content: None,
+        extension_data: Default::default(),
     }
 }