Backport: feat(ai): add convertDataPart option to convertToModelMessages

[vercel-ai-sdk]

This is an automated backport of #9720 to the release-v5.0 branch.
This backport has conflicts that need to be resolved manually.

git cherry-pick output

Auto-merging packages/ai/src/ui/convert-to-model-messages.test.ts
CONFLICT (content): Merge conflict in packages/ai/src/ui/convert-to-model-messages.test.ts
Auto-merging packages/ai/src/ui/convert-to-model-messages.ts
CONFLICT (content): Merge conflict in packages/ai/src/ui/convert-to-model-messages.ts
Auto-merging packages/ai/src/ui/index.ts
Auto-merging packages/ai/src/ui/ui-messages.ts
CONFLICT (content): Merge conflict in packages/ai/src/ui/ui-messages.ts
error: could not apply 9b8394786... feat(ai): add convertDataPart option to convertToModelMessages (#9720)
hint: After resolving the conflicts, mark them with
hint: "git add/rm <pathspec>", then run
hint: "git cherry-pick --continue".
hint: You can instead skip this commit with "git cherry-pick --skip".
hint: To abort and get back to the state before "git cherry-pick",
hint: run "git cherry-pick --abort".
hint: Disable this message with "git config set advice.mergeConflict false"