Message
@JsonSerializable
data class Message(val messageId: MessageId, val role: A2ARole, val parts: List<Part>, val contextId: ContextId? = null, val taskId: TaskId? = null, val metadata: Map<String, Any>? = null, val extensions: List<String>? = null, val referenceTaskIds: List<TaskId>? = null) : StreamItem, MessageResponse(source)