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/protocol/src/protocol.rs

@@ -438,6 +438,10 @@ pub enum EventMsg {
     /// Error while executing a submission
     Error(ErrorEvent),
 
+    /// Warning issued while processing a submission. Unlike `Error`, this
+    /// indicates the task continued but the user should still be notified.
+    Warning(WarningEvent),
+
     /// Agent has started a task
     TaskStarted(TaskStartedEvent),
 
@@ -672,6 +676,11 @@ pub struct ErrorEvent {
     pub message: String,
 }
 
+#[derive(Debug, Clone, Deserialize, Serialize, JsonSchema, TS)]
+pub struct WarningEvent {
+    pub message: String,
+}
+
 #[derive(Debug, Clone, Deserialize, Serialize, JsonSchema, TS)]
 pub struct TaskCompleteEvent {
     pub last_agent_message: Option<String>,