The two lookups every caller uses now honor Domain.directoryId, then the
server default, then the internal directory, so sign-in, bearer routing,
recipient lookup, discovery, the PACC record and the refusal of password
changes on external accounts all follow the domain. A directoryId, or a
server default, naming a directory that doesn't exist is unavailable,
never the internal directory.
A directory speaks only for the domains it serves: an account it returns
on another directory's domain is refused, for sign-in and recipients
alike, and aliases and group claims on such domains are dropped with a
warning. A bearer token must belong to the user the client names, or the
name must be one of its aliases with alias sign-in allowed. Accounts and
groups that sync creates pass the tenant checks, limits included.