Parse and expose stream errors (#2540)

codex-rs/protocol/src/protocol.rs

@@ -446,6 +446,10 @@ pub enum EventMsg {
 
     BackgroundEvent(BackgroundEventEvent),
 
+    /// Notification that a model stream experienced an error or disconnect
+    /// and the system is handling it (e.g., retrying with backoff).
+    StreamError(StreamErrorEvent),
+
     /// Notification that the agent is about to apply a code patch. Mirrors
     /// `ExecCommandBegin` so front‑ends can show progress indicators.
     PatchApplyBegin(PatchApplyBeginEvent),
@@ -721,6 +725,11 @@ pub struct BackgroundEventEvent {
     pub message: String,
 }
 
+#[derive(Debug, Clone, Deserialize, Serialize)]
+pub struct StreamErrorEvent {
+    pub message: String,
+}
+
 #[derive(Debug, Clone, Deserialize, Serialize)]
 pub struct PatchApplyBeginEvent {
     /// Identifier so this can be paired with the PatchApplyEnd event.