agent: Show delete thread icon buttons on hover/focus (#30370)

This PR's main goal is to show the delete thread button when the list
item is either focused or hovered. In order to do that, we ended up
refactoring (i.e., merging) the `PastThread` and `PastContext` elements
into a single `HistoryElementEntry` that already matches to the entry
type (i.e., context or thread).

Release Notes:

- agent: Simplify the UI by showing the delete thread icon button only
on hover or focus.

---------

Co-authored-by: Agus Zubiaga <hi@aguz.me>

crates/ui/src/components/list/list_item.rs

@@ -33,6 +33,7 @@ pub struct ListItem {
     toggle: Option<bool>,
     inset: bool,
     on_click: Option<Box<dyn Fn(&ClickEvent, &mut Window, &mut App) + 'static>>,
+    on_hover: Option<Box<dyn Fn(&bool, &mut Window, &mut App) + 'static>>,
     on_toggle: Option<Arc<dyn Fn(&ClickEvent, &mut Window, &mut App) + 'static>>,
     tooltip: Option<Box<dyn Fn(&mut Window, &mut App) -> AnyView + 'static>>,
     on_secondary_mouse_down: Option<Box<dyn Fn(&MouseDownEvent, &mut Window, &mut App) + 'static>>,
@@ -63,6 +64,7 @@ impl ListItem {
             on_click: None,
             on_secondary_mouse_down: None,
             on_toggle: None,
+            on_hover: None,
             tooltip: None,
             children: SmallVec::new(),
             selectable: true,
@@ -102,6 +104,11 @@ impl ListItem {
         self
     }
 
+    pub fn on_hover(mut self, handler: impl Fn(&bool, &mut Window, &mut App) + 'static) -> Self {
+        self.on_hover = Some(Box::new(handler));
+        self
+    }
+
     pub fn on_secondary_mouse_down(
         mut self,
         handler: impl Fn(&MouseDownEvent, &mut Window, &mut App) + 'static,
@@ -233,6 +240,7 @@ impl RenderOnce for ListItem {
                     })
             })
             .when(self.rounded, |this| this.rounded_sm())
+            .when_some(self.on_hover, |this, on_hover| this.on_hover(on_hover))
             .child(
                 h_flex()
                     .id("inner_list_item")