feat(core): export LegacyContextFields type from public surface

Allows Protocol subclasses (e.g. ext-apps) overriding buildContext()
to type their return as BaseContext & LegacyContextFields.

Author: felixweinberger

packages/core/src/exports/public/index.ts

@@ -42,6 +42,7 @@ export { getDisplayName } from '../../shared/metadataUtils.js';
 export type {
     BaseContext,
     ClientContext,
+    LegacyContextFields,
     NotificationOptions,
     ProgressCallback,
     ProtocolOptions,