chore(ai): export ContentPart type (#10902)

## Background

`onStepFinish` allows retrieval of `content` which is typed as
`ContentPart<ToolSet>[]`, and it was not possible to import the
`ContentPart` type from `ai`

## Summary

export `ContentPart`

Author: qkdreyer

.changeset/nine-frogs-exist.md

@@ -0,0 +1,5 @@
+---
+'ai': patch
+---
+
+chore(ai): export ContentPart type

packages/ai/src/generate-text/index.ts

@@ -3,6 +3,7 @@ export {
   type GenerateTextOnFinishCallback,
   type GenerateTextOnStepFinishCallback,
 } from './generate-text';
+export type { ContentPart } from './content-part';
 export type { GenerateTextResult } from './generate-text-result';
 export type {
   GeneratedFile as Experimental_GeneratedImage, // Image for backwards compatibility, TODO remove in v5