Follow-up to the group-DM manifest fix. The manifest change only helps
NEW installs; existing apps keep their old (mpim-less) scopes until the
admin reinstalls. Since a missing message.mpim event delivers nothing
(no runtime API error to catch), detect stale installs at connect time
from the auth.test x-oauth-scopes header and log an actionable reinstall
nudge when im:history is granted but mpim:history is not. Also promote
message.mpim from Recommended to Required in the docs event tables so the
default setup path can't drop it.
