Fix ACP connection and thread leak (#35670)

When you switched away from an ACP thread, the `AcpThreadView` entity
(and thus thread, and subprocess) was leaked. This happened because we
were using `cx.processor` for the `list` state callback, which uses a
strong reference.

This PR changes the callback so that it holds a weak reference, and adds
some tests and assertions at various levels to make sure we don't
reintroduce the leak in the future.

Release Notes:

- N/A

crates/agent_ui/src/agent_panel.rs

@@ -970,13 +970,7 @@ impl AgentPanel {
                     )
                 });
 
-                this.set_active_view(
-                    ActiveView::ExternalAgentThread {
-                        thread_view: thread_view.clone(),
-                    },
-                    window,
-                    cx,
-                );
+                this.set_active_view(ActiveView::ExternalAgentThread { thread_view }, window, cx);
             })
         })
         .detach_and_log_err(cx);
@@ -1477,6 +1471,7 @@ 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, .. } => {
@@ -1488,6 +1483,9 @@ impl AgentPanel {
                     });
                 }
             }
+            ActiveView::ExternalAgentThread { thread_view } => {
+                old_acp_thread.replace(thread_view.downgrade());
+            }
             _ => {}
         }
 
@@ -1518,6 +1516,11 @@ 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);