Add warning on compact (#6052)

This PR introduces the ability for `core` to send `warnings` as it can
send `errors. It also sends a warning on compaction.

<img width="811" height="187" alt="image"
src="https://github.com/user-attachments/assets/0947a42d-b720-420d-b7fd-115f8a65a46a"
/>

codex-rs/core/src/rollout/policy.rs

@@ -46,6 +46,7 @@ pub(crate) fn should_persist_event_msg(ev: &EventMsg) -> bool {
         | EventMsg::UndoCompleted(_)
         | EventMsg::TurnAborted(_) => true,
         EventMsg::Error(_)
+        | EventMsg::Warning(_)
         | EventMsg::TaskStarted(_)
         | EventMsg::TaskComplete(_)
         | EventMsg::AgentMessageDelta(_)