fix: session downgrade (#8196)

The problem is that the `tokio` task own an `Arc` reference of the
session and that this task only exit with the broadcast channel get
closed. But this never get closed if the session is not dropped. So it's
a snake biting his tail basically

The most notable result was that non of the `Drop` implementation were
triggered (temporary files, shell snapshots, session cleaning etc etc)
when closing the session (through a `/new` for example)

The fix is just to weaken the `Arc` and upgrade it on the fly

codex-rs/core/src/codex.rs

@@ -738,11 +738,14 @@ impl Session {
                 .services
                 .skills_manager
                 .subscribe_skills_update_notifications();
-            let sess = Arc::clone(&sess);
+            let sess = Arc::downgrade(&sess);
             tokio::spawn(async move {
                 loop {
                     match rx.recv().await {
                         Ok(()) => {
+                            let Some(sess) = sess.upgrade() else {
+                                break;
+                            };
                             let turn_context = sess.new_default_turn().await;
                             sess.send_event(turn_context.as_ref(), EventMsg::SkillsUpdateAvailable)
                                 .await;