assistant2: Decouple scripting tool from the `Tool` trait (#26382)

This PR decouples the scripting tool from the `Tool` trait while still
allowing it to be used as a tool from the model's perspective.

This will allow us to evolve the scripting tool as more of a first-class
citizen while still retaining the ability to have the model call it as a
regular tool.

Release Notes:

- N/A