agent_ui: Iterate on title display on the toolbar (#59162)

Follow up to https://github.com/zed-industries/zed/pull/59126 where
seeing the typed-prompt in the agent panel's toolbar felt really busy.
With that change, I'd be seeing the prompt I'm writing in three
different stops, which felt overwhelming. Toning that down a little bit
in here.

Release Notes:

- N/A

crates/agent_ui/src/agent_panel.rs

@@ -1178,7 +1178,6 @@ pub struct AgentPanel {
     _base_view_observation: Option<Subscription>,
     _draft_editor_observation: Option<Subscription>,
     _active_draft_reclaim_observation: Option<Subscription>,
-    _empty_thread_title_observation: Option<Subscription>,
     _thread_metadata_store_subscription: Subscription,
     last_context_source: Option<AgentContextSource>,
 
@@ -1586,7 +1585,6 @@ impl AgentPanel {
             _base_view_observation: None,
             _draft_editor_observation: None,
             _active_draft_reclaim_observation: None,
-            _empty_thread_title_observation: None,
             _thread_metadata_store_subscription,
             last_context_source: None,
             is_active: false,
@@ -3015,28 +3013,6 @@ impl AgentPanel {
             }));
     }
 
-    /// Re-renders the toolbar as the user types so the empty-thread
-    /// placeholder title tracks the message editor's content.
-    fn observe_message_editor_for_toolbar_title(
-        &mut self,
-        conversation_view: &Entity<ConversationView>,
-        cx: &mut Context<Self>,
-    ) {
-        let Some(editor) = conversation_view
-            .read(cx)
-            .root_thread_view()
-            .map(|tv| tv.read(cx).message_editor.clone())
-        else {
-            self._empty_thread_title_observation = None;
-            return;
-        };
-        self._empty_thread_title_observation = Some(cx.observe(&editor, |this, _editor, cx| {
-            if !this.active_thread_has_messages(cx) {
-                cx.notify();
-            }
-        }));
-    }
-
     fn try_make_empty_draft_ephemeral(
         &mut self,
         conversation_view: Entity<ConversationView>,
@@ -4247,12 +4223,10 @@ impl AgentPanel {
                     }));
                 let cv = conversation_view.clone();
                 self.observe_active_draft_for_empty_editor(&cv, cx);
-                self.observe_message_editor_for_toolbar_title(&cv, cx);
                 Some(cx.observe_in(&cv, window, |this, server_view, window, cx| {
                     this._thread_view_subscription =
                         Self::subscribe_to_active_thread_view(&server_view, window, cx);
                     this.observe_active_draft_for_empty_editor(&server_view, cx);
-                    this.observe_message_editor_for_toolbar_title(&server_view, cx);
                     cx.emit(AgentPanelEvent::ActiveViewChanged);
                     this.serialize(cx);
                     cx.notify();
@@ -4260,7 +4234,6 @@ impl AgentPanel {
             }
             BaseView::Terminal { terminal_id } => {
                 self._thread_view_subscription = None;
-                self._empty_thread_title_observation = None;
                 if let Some(terminal) = self.terminals.get(terminal_id) {
                     let terminal_id = *terminal_id;
                     let focus_handle = terminal.view.focus_handle(cx);
@@ -4282,7 +4255,6 @@ impl AgentPanel {
             BaseView::Uninitialized => {
                 self._thread_view_subscription = None;
                 self._active_thread_focus_subscription = None;
-                self._empty_thread_title_observation = None;
                 None
             }
         };
@@ -6122,21 +6094,12 @@ impl AgentPanel {
             .flex_none()
             .justify_between();
 
-        let empty_thread_title = if matches!(mode, ToolbarMode::EmptyThread) {
-            let draft_label = self
-                .active_thread_id(cx)
-                .and_then(|thread_id| self.editor_text(thread_id, cx))
-                .and_then(|text| crate::draft_prompt_store::truncate_draft_label(&text));
-            Some(match draft_label {
-                Some(label) => Label::new(label).truncate().into_any_element(),
-                None => Label::new(format!("New {} Thread", selected_agent_label))
-                    .color(Color::Muted)
-                    .truncate()
-                    .into_any_element(),
-            })
-        } else {
-            None
-        };
+        let empty_thread_title = matches!(mode, ToolbarMode::EmptyThread).then(|| {
+            Label::new(format!("New {} Thread", selected_agent_label))
+                .color(Color::Muted)
+                .truncate()
+                .into_any_element()
+        });
 
         let toolbar_content = {
             let new_thread_menu = PopoverMenu::new("new_thread_menu")