feat (ai): export InferUIMessageChunk type (#7349)

## Background

Requested in #7347

## Summary

export InferUIMessageChunk type

## Related Issues

Closes #7347

Author: lgrammel

.changeset/clever-moles-arrive.md

@@ -0,0 +1,5 @@
+---
+'ai': patch
+---
+
+feat (ai): export InferUIMessageChunk type

packages/ai/src/ui-message-stream/index.ts

@@ -3,6 +3,6 @@ export { createUIMessageStreamResponse } from './create-ui-message-stream-respon
 export { JsonToSseTransformStream } from './json-to-sse-transform-stream';
 export { pipeUIMessageStreamToResponse } from './pipe-ui-message-stream-to-response';
 export { readUIMessageStream } from './read-ui-message-stream';
-export type { UIMessageChunk } from './ui-message-chunks';
+export type { InferUIMessageChunk, UIMessageChunk } from './ui-message-chunks';
 export { UI_MESSAGE_STREAM_HEADERS } from './ui-message-stream-headers';
 export type { UIMessageStreamWriter } from './ui-message-stream-writer';