assistant2: Factor out `ActiveThread` view (#21555)

This PR factors a new `ActiveThread` view out of the `AssistantPanel` to
group together the state that pertains solely to the active view.

There was a bunch of related state on the `AssistantPanel` pertaining to
the active thread that needed to be initialized/reset together and it
makes for a clearer narrative is this state is encapsulated in its own
view.

Release Notes:

- N/A