Export the legacy handler types from the root entry

They appear in App's public setRequestHandler / setNotificationHandler
signatures, so TypeDoc's link validation needs them reachable from the
entry point.

src/app.ts

@@ -30,6 +30,12 @@ import {
   type LegacyNotificationHandler,
   type LegacyRequestHandler,
 } from "./legacy-handlers.js";
+export type {
+  LegacyMethodSchema,
+  LegacyNotificationHandler,
+  LegacyRequestHandler,
+  LegacyRequestHandlerExtra,
+} from "./legacy-handlers.js";
 export { EventDispatcher } from "./events.js";
 
 import { PostMessageTransport } from "./message-transport.js";