Request body for updating a sections-based note.

When content_map is sent on a base note update, snapshot_id is required (from create or GET /notes/{id}/edit) for collaborative conflict detection.

Field order: content-update fields (content_map, snapshot_id) first, then mutable fields in the same order as ElationNoteCreate (provider through metadata; create-only layout_name, practice, patient omitted), then confidential.