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_servers/src/acp/v1.rs

@@ -19,7 +19,6 @@ pub struct AcpConnection {
     sessions: Rc<RefCell<HashMap<acp::SessionId, AcpSession>>>,
     auth_methods: Vec<acp::AuthMethod>,
     _io_task: Task<Result<()>>,
-    _child: smol::process::Child,
 }
 
 pub struct AcpSession {
@@ -47,6 +46,7 @@ impl AcpConnection {
 
         let stdout = child.stdout.take().expect("Failed to take stdout");
         let stdin = child.stdin.take().expect("Failed to take stdin");
+        log::trace!("Spawned (pid: {})", child.id());
 
         let sessions = Rc::new(RefCell::new(HashMap::default()));
 
@@ -61,7 +61,11 @@ impl AcpConnection {
             }
         });
 
-        let io_task = cx.background_spawn(io_task);
+        let io_task = cx.background_spawn(async move {
+            io_task.await?;
+            drop(child);
+            Ok(())
+        });
 
         let response = connection
             .initialize(acp::InitializeRequest {
@@ -84,7 +88,6 @@ impl AcpConnection {
             connection: connection.into(),
             server_name,
             sessions,
-            _child: child,
             _io_task: io_task,
         })
     }
@@ -155,8 +158,10 @@ impl AgentConnection for AcpConnection {
 
     fn prompt(&self, params: acp::PromptRequest, cx: &mut App) -> Task<Result<()>> {
         let conn = self.connection.clone();
-        cx.foreground_executor()
-            .spawn(async move { Ok(conn.prompt(params).await?) })
+        cx.foreground_executor().spawn(async move {
+            conn.prompt(params).await?;
+            Ok(())
+        })
     }
 
     fn cancel(&self, session_id: &acp::SessionId, cx: &mut App) {