Final event for a turn, optionally carrying final response and usage data.
@type t() :: %Codex.Events.TurnCompleted{ error: map() | nil, final_response: Codex.Items.AgentMessage.t() | map() | nil, response_id: String.t() | nil, status: String.t() | nil, thread_id: String.t() | nil, turn_id: String.t() | nil, usage: map() | nil }