assistant2: Only show thread context in picker when we have a `ThreadStore` (#23501)

This PR fixes an issue with the context picker where it would show
thread context as an option even if there was no `ThreadStore`
available.

Release Notes:

- N/A
01b57b10e7Marshall Bowers committed on 1/22/2025, 11:00:43 PM· committed by GitHubparenta1077c6
2 files changedLine totals unavailable