Simplify ListState API (#35685)

Follow up to: https://github.com/zed-industries/zed/pull/35670,
simplifies the List state APIs so you no longer have to worry about
strong vs. weak pointers when rendering list items.

Release Notes:

- N/A

---------

Co-authored-by: Agus Zubiaga <agus@zed.dev>

crates/agent_ui/src/agent_panel.rs

@@ -1471,7 +1471,6 @@ impl AgentPanel {
 
         let current_is_special = current_is_history || current_is_config;
         let new_is_special = new_is_history || new_is_config;
-        let mut old_acp_thread = None;
 
         match &self.active_view {
             ActiveView::Thread { thread, .. } => {
@@ -1483,9 +1482,6 @@ impl AgentPanel {
                     });
                 }
             }
-            ActiveView::ExternalAgentThread { thread_view } => {
-                old_acp_thread.replace(thread_view.downgrade());
-            }
             _ => {}
         }
 
@@ -1516,11 +1512,6 @@ impl AgentPanel {
             self.active_view = new_view;
         }
 
-        debug_assert!(
-            old_acp_thread.map_or(true, |thread| !thread.is_upgradable()),
-            "AcpThreadView leaked"
-        );
-
         self.acp_message_history.borrow_mut().reset_position();
 
         self.focus_handle(cx).focus(window);