Protoface session ID after start() succeeds, otherwise null.
Start the avatar session.
Subclasses should override this method and call super.start(agentSession, room) at the
top of their implementation. Subclasses may widen the return type (e.g. returning a
session id), matching the # type: ignore[override] escape hatch used in Python.
A Protoface avatar session for LiveKit Agents.