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

This reverts commit c2ec477d93.

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codex-rs/exec/tests/event_processor_with_json_output.rs

@@ -539,7 +539,6 @@ fn error_event_produces_error() {
         "e1",
         EventMsg::Error(codex_core::protocol::ErrorEvent {
             message: "boom".to_string(),
-            http_status_code: Some(500),
         }),
     ));
     assert_eq!(
@@ -579,7 +578,6 @@ fn stream_error_event_produces_error() {
         "e1",
         EventMsg::StreamError(codex_core::protocol::StreamErrorEvent {
             message: "retrying".to_string(),
-            http_status_code: Some(500),
         }),
     ));
     assert_eq!(
@@ -598,7 +596,6 @@ fn error_followed_by_task_complete_produces_turn_failed() {
         "e1",
         EventMsg::Error(ErrorEvent {
             message: "boom".to_string(),
-            http_status_code: Some(500),
         }),
     );
     assert_eq!(