tab_switcher: Add placeholder text (#31697)

| Before | After |
|---|---|
|<img width="478" alt="image"
src="https://github.com/user-attachments/assets/5baba783-ee31-42cd-9760-7ee19edb1123"
/>|<img width="478" alt="image"
src="https://github.com/user-attachments/assets/1b149500-4a97-4085-80e5-fd628c92471a"
/>|

Release Notes:

- N/A

crates/tab_switcher/src/tab_switcher.rs

@@ -450,7 +450,7 @@ impl PickerDelegate for TabSwitcherDelegate {
     type ListItem = ListItem;
 
     fn placeholder_text(&self, _window: &mut Window, _cx: &mut App) -> Arc<str> {
-        Arc::default()
+        "Search all tabs…".into()
     }
 
     fn no_matches_text(&self, _window: &mut Window, _cx: &mut App) -> Option<SharedString> {