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/thread.rs

@@ -85,6 +85,10 @@ impl Thread {
         &self.id
     }
 
+    pub fn is_empty(&self) -> bool {
+        self.messages.is_empty()
+    }
+
     pub fn message(&self, id: MessageId) -> Option<&Message> {
         self.messages.iter().find(|message| message.id == id)
     }