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/tests/event_processor_with_json_output.rs

@@ -12,11 +12,13 @@ use codex_core::protocol::McpToolCallEndEvent;
 use codex_core::protocol::PatchApplyBeginEvent;
 use codex_core::protocol::PatchApplyEndEvent;
 use codex_core::protocol::SessionConfiguredEvent;
+use codex_core::protocol::WarningEvent;
 use codex_core::protocol::WebSearchEndEvent;
 use codex_exec::event_processor_with_jsonl_output::EventProcessorWithJsonOutput;
 use codex_exec::exec_events::AgentMessageItem;
 use codex_exec::exec_events::CommandExecutionItem;
 use codex_exec::exec_events::CommandExecutionStatus;
+use codex_exec::exec_events::ErrorItem;
 use codex_exec::exec_events::ItemCompletedEvent;
 use codex_exec::exec_events::ItemStartedEvent;
 use codex_exec::exec_events::ItemUpdatedEvent;
@@ -540,6 +542,28 @@ fn error_event_produces_error() {
     );
 }
 
+#[test]
+fn warning_event_produces_error_item() {
+    let mut ep = EventProcessorWithJsonOutput::new(None);
+    let out = ep.collect_thread_events(&event(
+        "e1",
+        EventMsg::Warning(WarningEvent {
+            message: "Heads up: Long conversations and multiple compactions can cause the model to be less accurate. Start new a new conversation when possible to keep conversations small and targeted.".to_string(),
+        }),
+    ));
+    assert_eq!(
+        out,
+        vec![ThreadEvent::ItemCompleted(ItemCompletedEvent {
+            item: ThreadItem {
+                id: "item_0".to_string(),
+                details: ThreadItemDetails::Error(ErrorItem {
+                    message: "Heads up: Long conversations and multiple compactions can cause the model to be less accurate. Start new a new conversation when possible to keep conversations small and targeted.".to_string(),
+                }),
+            },
+        })]
+    );
+}
+
 #[test]
 fn stream_error_event_produces_error() {
     let mut ep = EventProcessorWithJsonOutput::new(None);