Structures supported:
a) agents_dir/agent_name.py (with root_agent or agent.root_agent in it)
b) agents_dir/agent_name_folder/__init__.py (with root_agent or agent.root_agent in the package)
c) agents_dir/agent_name_folder/agent.py (where agent.py has root_agent)
PiperOrigin-RevId: 763943716