The active client session. Exposes the SessionState
surface — including the lists of active and inbound calls — once the
client is connected and authenticated.
This is the entry point for inbound calls: subscribe to
session.incomingCalls$
after calling register()
to receive notification of ringing calls.
SessionState — full interface, including calls$, incomingCalls$, authenticated$.register — must be called before inbound calls route to this session.