@type part() ::
  ExAgent.Message.Part.System.t()
  | ExAgent.Message.Part.User.t()
  | ExAgent.Message.Part.ToolReturn.t()
  | ExAgent.Message.Part.Retry.t()

@type t() :: %ExAgent.Message.Request{
  conversation_id: String.t() | nil,
  instructions: String.t() | nil,
  parts: [part()],
  run_id: String.t() | nil,
  timestamp: DateTime.t() | nil
}