agent: Make ToolWorkingSet an Entity (#28757)

Motivation is to emit events when enabled tools change, want to use this
in #28755

Release Notes:

- N/A

crates/agent/src/assistant_panel.rs

@@ -203,7 +203,7 @@ impl AssistantPanel {
         cx: AsyncWindowContext,
     ) -> Task<Result<Entity<Self>>> {
         cx.spawn(async move |cx| {
-            let tools = Arc::new(ToolWorkingSet::default());
+            let tools = cx.new(|_| ToolWorkingSet::default())?;
             let thread_store = workspace
                 .update(cx, |workspace, cx| {
                     let project = workspace.project().clone();