Retrieve live session

Returns the current observable payer-channel session, when available. Use this endpoint while `status = processing` to show a read-only view of the current payer-channel session, or while manual handoff is active to understand whether interactive access is available. `LiveSession` is a transparency surface. It is not payer receipt, payer decision, or requirement truth. It never returns raw runtime-provider access URLs; create a live-session access grant when `access.available = true`. ## Returns Returns the current live-session projection. If no session has started, returns `state = not_live` with `200 OK`.