refactor: remove redundant export of MessagePart and MessagePartType

app/composables/useAgentChat.ts

@@ -4,8 +4,6 @@ import { useAuthSession } from "./useAuthSession";
 import { useAgentRateLimit } from "./useAgentRateLimit";
 import type { MessagePart, MessagePartType } from "../types/chat";
 
-export type { MessagePart, MessagePartType } from "../types/chat";
-
 export interface AgentMessage {
   id: string;
   role: "user" | "assistant";