<!-- CURSOR_SUMMARY -->
> [!NOTE]
> Updates AI SDK imports to use the top-level `ai` package instead of
`node_modules/ai/dist`.
> 
> - In `prepare_step_utils.ts` and its tests, replace imports of
`ImagePart`, `ModelMessage`, `TextPart`, and `UserModelMessage` to come
from `ai`
> - No functional code changes beyond import paths
> 
> <sup>Written by [Cursor
Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit
117078577aa8f7a86564f8a133c2708601f4acb1. This will update automatically
on new commits. Configure
[here](https://cursor.com/dashboard?tab=bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

<!-- This is an auto-generated description by cubic. -->
## Summary by cubic
Replaced deep imports from node_modules/ai/dist with the ai package
entrypoint to standardize and simplify module imports. This avoids
brittle paths and improves build compatibility.

<sup>Written for commit 117078577aa8f7a86564f8a133c2708601f4acb1.
Summary will update on new commits.</sup>

<!-- End of auto-generated description by cubic. -->