Optionalrequested: string | null
The call's or session's backend; auto or absent for the configured route.
eligible narrows an auto route; config supplies an existing snapshot or configuration.
The configured default is kept when eligible; otherwise the
route chooses among the eligible backends. An explicit backend is never replaced.
Resolves the backend of a turn: an explicit backend, else the configured default, else one chosen like smart_quota among the permitted installed CLIs.