Replaces deprecated ai_client.send(md_content='', user_message=user_message,
enable_tools=False) with ai_client.send_result(...) and branches on
result.ok. On error, logs the ui_message() and returns [] (the function
returns a list of track definitions or [] on failure).
The 3 tests in test_orchestrator_pm.py + 1 in test_orchestrator_pm_history.py
now fail because they mock src.ai_client.send. These will be fixed in
Phase 2.14-2.15 by mocking send_result instead.