BACK-2605: read-only preview for the "group by Department" UI flow — returns the direct sub-teams of parentOrgUnitId and their count, for the "this will independently cap N teams" confirmation copy. Creates nothing: the actual budget is a single POST /v2/api/ai/cost-controls write with groupBy: "ORG_UNIT", which already gives each org unit its own independent cap against the full hardLimit — mirroring how groupBy: "SUBSCRIBER" already compares each subscriber's own balance against the full threshold rather than a shared pool. Requires org-unit-budgets-enabled AND org-unit-attribution-enabled for the team; inert (422) otherwise.