assistant: Use tool interface for scripts (#26377)

We decided to expose scripting as tools again. We are aware of the UX
downsides of doing so, but we want to focus on getting it working well
first, and the model seems to make better use of it as an actual tool.

In the future, the tools API might support streaming. If it doesn't and
we need to ship, we can consider reverting this.

Release Notes:

- N/A