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

crates/assistant2/src/assistant.rs

@@ -1,3 +1,4 @@
+mod active_thread;
 mod assistant_panel;
 mod message_editor;
 mod thread;