Hands the channel the session's MJ data context. Called exactly once per session by the host
between Initialize(ctx) and OnSessionStarted(). Implementations should just stash the
values for use in their own OnSessionStarted().
The user the realtime session is running as (for RunView / entity loads).
The metadata provider the session authenticated on (multi-provider safe).
Implemented by server-side channel plugins that need MJ data access for their session-start work. The host calls SetSessionDataContext once, after
Initialize(ctx)and beforeOnSessionStarted(), with the SAMEcontextUser+providerthe session authenticated on.