[app-server & core] introduce new codex error code and v2 app-server error events (#6938)

This PR does two things:
1. populate a new `codex_error_code` protocol in error events sent from
core to client;
2. old v1 core events `codex/event/stream_error` and `codex/event/error`
will now both become `error`. We also show codex error code for
turncompleted -> error status.

new events in app server test:
```
< {
<   "method": "codex/event/stream_error",
<   "params": {
<     "conversationId": "019aa34c-0c14-70e0-9706-98520a760d67",
<     "id": "0",
<     "msg": {
<       "codex_error_code": {
<         "response_stream_disconnected": {
<           "http_status_code": 401
<         }
<       },
<       "message": "Reconnecting... 2/5",
<       "type": "stream_error"
<     }
<   }
< }

 {
<   "method": "error",
<   "params": {
<     "error": {
<       "codexErrorCode": {
<         "responseStreamDisconnected": {
<           "httpStatusCode": 401
<         }
<       },
<       "message": "Reconnecting... 2/5"
<     }
<   }
< }

< {
<   "method": "turn/completed",
<   "params": {
<     "turn": {
<       "error": {
<         "codexErrorCode": {
<           "responseTooManyFailedAttempts": {
<             "httpStatusCode": 401
<           }
<         },
<         "message": "exceeded retry limit, last status: 401 Unauthorized, request id: 9a1b495a1a97ed3e-SJC"
<       },
<       "id": "0",
<       "items": [],
<       "status": "failed"
<     }
<   }
< }
```

codex-rs/core/src/error.rs

@@ -10,6 +10,8 @@ use chrono::Local;
 use chrono::Utc;
 use codex_async_utils::CancelErr;
 use codex_protocol::ConversationId;
+use codex_protocol::protocol::CodexErrorInfo;
+use codex_protocol::protocol::ErrorEvent;
 use codex_protocol::protocol::RateLimitSnapshot;
 use reqwest::StatusCode;
 use serde_json;
@@ -430,6 +432,57 @@ impl CodexErr {
     pub fn downcast_ref<T: std::any::Any>(&self) -> Option<&T> {
         (self as &dyn std::any::Any).downcast_ref::<T>()
     }
+
+    /// Translate core error to client-facing protocol error.
+    pub fn to_codex_protocol_error(&self) -> CodexErrorInfo {
+        match self {
+            CodexErr::ContextWindowExceeded => CodexErrorInfo::ContextWindowExceeded,
+            CodexErr::UsageLimitReached(_)
+            | CodexErr::QuotaExceeded
+            | CodexErr::UsageNotIncluded => CodexErrorInfo::UsageLimitExceeded,
+            CodexErr::RetryLimit(_) => CodexErrorInfo::ResponseTooManyFailedAttempts {
+                http_status_code: self.http_status_code_value(),
+            },
+            CodexErr::ConnectionFailed(_) => CodexErrorInfo::HttpConnectionFailed {
+                http_status_code: self.http_status_code_value(),
+            },
+            CodexErr::ResponseStreamFailed(_) => CodexErrorInfo::ResponseStreamConnectionFailed {
+                http_status_code: self.http_status_code_value(),
+            },
+            CodexErr::RefreshTokenFailed(_) => CodexErrorInfo::Unauthorized,
+            CodexErr::SessionConfiguredNotFirstEvent
+            | CodexErr::InternalServerError
+            | CodexErr::InternalAgentDied => CodexErrorInfo::InternalServerError,
+            CodexErr::UnsupportedOperation(_) | CodexErr::ConversationNotFound(_) => {
+                CodexErrorInfo::BadRequest
+            }
+            CodexErr::Sandbox(_) => CodexErrorInfo::SandboxError,
+            _ => CodexErrorInfo::Other,
+        }
+    }
+
+    pub fn to_error_event(&self, message_prefix: Option<String>) -> ErrorEvent {
+        let error_message = self.to_string();
+        let message: String = match message_prefix {
+            Some(prefix) => format!("{prefix}: {error_message}"),
+            None => error_message,
+        };
+        ErrorEvent {
+            message,
+            codex_error_code: Some(self.to_codex_protocol_error()),
+        }
+    }
+
+    pub fn http_status_code_value(&self) -> Option<u16> {
+        let http_status_code = match self {
+            CodexErr::RetryLimit(err) => Some(err.status),
+            CodexErr::UnexpectedStatus(err) => Some(err.status),
+            CodexErr::ConnectionFailed(err) => err.source.status(),
+            CodexErr::ResponseStreamFailed(err) => err.source.status(),
+            _ => None,
+        };
+        http_status_code.as_ref().map(StatusCode::as_u16)
+    }
 }
 
 pub fn get_error_message_ui(e: &CodexErr) -> String {
@@ -478,6 +531,10 @@ mod tests {
     use chrono::Utc;
     use codex_protocol::protocol::RateLimitWindow;
     use pretty_assertions::assert_eq;
+    use reqwest::Response;
+    use reqwest::ResponseBuilderExt;
+    use reqwest::StatusCode;
+    use reqwest::Url;
 
     fn rate_limit_snapshot() -> RateLimitSnapshot {
         let primary_reset_at = Utc
@@ -573,6 +630,33 @@ mod tests {
         assert_eq!(get_error_message_ui(&err), "stdout only");
     }
 
+    #[test]
+    fn to_error_event_handles_response_stream_failed() {
+        let response = http::Response::builder()
+            .status(StatusCode::TOO_MANY_REQUESTS)
+            .url(Url::parse("http://example.com").unwrap())
+            .body("")
+            .unwrap();
+        let source = Response::from(response).error_for_status_ref().unwrap_err();
+        let err = CodexErr::ResponseStreamFailed(ResponseStreamFailed {
+            source,
+            request_id: Some("req-123".to_string()),
+        });
+
+        let event = err.to_error_event(Some("prefix".to_string()));
+
+        assert_eq!(
+            event.message,
+            "prefix: Error while reading the server response: HTTP status client error (429 Too Many Requests) for url (http://example.com/), request id: req-123"
+        );
+        assert_eq!(
+            event.codex_error_code,
+            Some(CodexErrorInfo::ResponseStreamConnectionFailed {
+                http_status_code: Some(429)
+            })
+        );
+    }
+
     #[test]
     fn sandbox_denied_reports_exit_code_when_no_output_available() {
         let output = ExecToolCallOutput {