agent_ui: Fix "scroll to" buttons (#53232)

Follow-up to https://github.com/zed-industries/zed/pull/53101

In the process of fixing the thread view's scroll experience, we for got
to turn off the follow state tail for functions that power the scroll
buttons in the agent panel.

Release Notes:

- N/A

crates/gpui/src/elements/list.rs

@@ -462,6 +462,13 @@ impl ListState {
 
         let current_offset = self.logical_scroll_top();
         let state = &mut *self.0.borrow_mut();
+
+        if distance < px(0.) {
+            if let FollowState::Tail { is_following } = &mut state.follow_state {
+                *is_following = false;
+            }
+        }
+
         let mut cursor = state.items.cursor::<ListItemSummary>(());
         cursor.seek(&Count(current_offset.item_ix), Bias::Right);
 
@@ -536,6 +543,12 @@ impl ListState {
             scroll_top.offset_in_item = px(0.);
         }
 
+        if scroll_top.item_ix < item_count {
+            if let FollowState::Tail { is_following } = &mut state.follow_state {
+                *is_following = false;
+            }
+        }
+
         state.logical_scroll_top = Some(scroll_top);
     }