Add session method coverage tests

- Add test_set_task_result_handler for line 166
- Add test_build_elicit_request for lines 531-547
- Add test_build_create_message_request for lines 583-605
- Add test_send_message for line 625
- Add test_response_routing_success for shared/session.py line 481-482
- Add test_response_routing_error for shared/session.py line 475-476
- Use Events instead of sleeps for deterministic async synchronization

Author: maxisbey