assistant2: Push logic for adding thread context down into the `ContextStore` (#22855)

This PR takes the logic for adding thread context out of the
`ThreadContextPicker` and pushes it down into the `ContextStore`.

Release Notes:

- N/A
7e39023ea5Marshall Bowers committed on 1/8/2025, 7:54:54 PM· committed by GitHubparentb783965
2 files changedLine totals unavailable