agent: Dismiss the agent panel notification if window is closed (#34230)

Closes https://github.com/zed-industries/zed/issues/32951

Release Notes:

- agent: Fixed an issue where the agent panel notification would linger
on even after you closed the window.

crates/agent_ui/src/active_thread.rs

@@ -787,6 +787,15 @@ impl ActiveThread {
                     .unwrap()
             }
         });
+
+        let workspace_subscription = if let Some(workspace) = workspace.upgrade() {
+            Some(cx.observe_release(&workspace, |this, _, cx| {
+                this.dismiss_notifications(cx);
+            }))
+        } else {
+            None
+        };
+
         let mut this = Self {
             language_registry,
             thread_store,
@@ -834,6 +843,10 @@ impl ActiveThread {
             }
         }
 
+        if let Some(subscription) = workspace_subscription {
+            this._subscriptions.push(subscription);
+        }
+
         this
     }