Revert "[core] add optional status_code to error events (#6865)" (#6955)

This reverts commit c2ec477d93.

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codex-rs/tui/src/chatwidget/tests.rs

@@ -2596,7 +2596,6 @@ fn stream_error_updates_status_indicator() {
         id: "sub-1".into(),
         msg: EventMsg::StreamError(StreamErrorEvent {
             message: msg.to_string(),
-            http_status_code: None,
         }),
     });