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/exec/src/event_processor_with_jsonl_output.rs

@@ -8,6 +8,7 @@ use crate::event_processor::handle_last_message;
 use crate::exec_events::AgentMessageItem;
 use crate::exec_events::CommandExecutionItem;
 use crate::exec_events::CommandExecutionStatus;
+use crate::exec_events::ErrorItem;
 use crate::exec_events::FileChangeItem;
 use crate::exec_events::FileUpdateChange;
 use crate::exec_events::ItemCompletedEvent;
@@ -129,6 +130,15 @@ impl EventProcessorWithJsonOutput {
                 self.last_critical_error = Some(error.clone());
                 vec![ThreadEvent::Error(error)]
             }
+            EventMsg::Warning(ev) => {
+                let item = ThreadItem {
+                    id: self.get_next_item_id(),
+                    details: ThreadItemDetails::Error(ErrorItem {
+                        message: ev.message.clone(),
+                    }),
+                };
+                vec![ThreadEvent::ItemCompleted(ItemCompletedEvent { item })]
+            }
             EventMsg::StreamError(ev) => vec![ThreadEvent::Error(ThreadErrorEvent {
                 message: ev.message.clone(),
             })],