A text segment within a message or artifact.
Wire shape: {"kind": "text", "text": <string>, "metadata": <map?>}.
{"kind": "text", "text": <string>, "metadata": <map?>}
@type t() :: %A2aEngine.Types.TextPart{metadata: map() | nil, text: String.t()}
@spec from_map(map()) :: {:ok, t()} | {:error, term()}
@spec to_map(t()) :: map()