Returns one row per unique session_number in the cohort's group_coaching modality, with a has_authored_content flag so the picker can grey out empty candidates (AC-EP3). The session_id is a representative row at that session_number — pass it verbatim back to POST .../agenda/duplicate-from as source_session_id; the service resolves to the canonical (cohort, modality, session_number) scope, identical for every per-group row at the same session_number.

Empty array when the cohort has no group_coaching modality or no sessions with a session_number — the picker handles "nothing to choose" UI-side.