Creates one or more agents on the calling credential's team from canonical row dicts, applying
the same license-limit, group, and role logic as the Classic back-office CSV import and the
Sunburst import_agents chat tool. Each row is processed independently -- a failure on one row
(invalid email, duplicate email, etc.) does not prevent the others from being created.

Only team admins may call this route. An agent-level credential must belong to a team admin
(RequiredAccess.ADMIN_ONLY); team and system credentials are privileged by construction and
always satisfy this check. System credentials target a team via the sisu-team-id header.