`account.scope` is now treated as the user's accumulated granted-scope
set rather than the latest token's `scope` claim. linkSocial unions
newly granted scopes into the stored value, while sign-in
re-authentication and refresh-token responses leave it untouched.
Generalizes the Google-specific `include_granted_scopes=true` band-aid
from #3013 into a provider-agnostic invariant grounded in RFC 6749 §1.5.