fix: export type UseStream (#1882)

Author: andrewnguonly

.changeset/many-mangos-know.md

@@ -0,0 +1,5 @@
+---
+"@langchain/langgraph-sdk": patch
+---
+
+Fix bug in UseStream export.

libs/sdk/src/react/index.ts

@@ -1,6 +1,6 @@
 export { useStream } from "./stream.js";
 export { FetchStreamTransport } from "./stream.custom.js";
-export { UseStream, UseStreamCustom } from "./types.js";
+export type { UseStream, UseStreamCustom } from "./types.js";
 export type {
   MessageMetadata,
   UseStreamOptions,