[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/app-server-protocol/src/protocol/v2.rs

@@ -11,6 +11,7 @@ use codex_protocol::items::AgentMessageContent as CoreAgentMessageContent;
 use codex_protocol::items::TurnItem as CoreTurnItem;
 use codex_protocol::models::ResponseItem;
 use codex_protocol::parse_command::ParsedCommand as CoreParsedCommand;
+use codex_protocol::protocol::CodexErrorInfo as CoreCodexErrorInfo;
 use codex_protocol::protocol::CreditsSnapshot as CoreCreditsSnapshot;
 use codex_protocol::protocol::RateLimitSnapshot as CoreRateLimitSnapshot;
 use codex_protocol::protocol::RateLimitWindow as CoreRateLimitWindow;
@@ -20,6 +21,7 @@ use schemars::JsonSchema;
 use serde::Deserialize;
 use serde::Serialize;
 use serde_json::Value as JsonValue;
+use thiserror::Error;
 use ts_rs::TS;
 
 // Macro to declare a camelCased API v2 enum mirroring a core enum which
@@ -47,6 +49,69 @@ macro_rules! v2_enum_from_core {
     };
 }
 
+/// This translation layer make sure that we expose codex error code in camel case.
+#[derive(Serialize, Deserialize, Debug, Clone, PartialEq, Eq, JsonSchema, TS)]
+#[serde(rename_all = "camelCase")]
+#[ts(export_to = "v2/")]
+pub enum CodexErrorInfo {
+    ContextWindowExceeded,
+    UsageLimitExceeded,
+    HttpConnectionFailed {
+        #[serde(rename = "httpStatusCode")]
+        #[ts(rename = "httpStatusCode")]
+        http_status_code: Option<u16>,
+    },
+    /// Failed to connect to the response SSE stream.
+    ResponseStreamConnectionFailed {
+        #[serde(rename = "httpStatusCode")]
+        #[ts(rename = "httpStatusCode")]
+        http_status_code: Option<u16>,
+    },
+    InternalServerError,
+    Unauthorized,
+    BadRequest,
+    SandboxError,
+    /// The response SSE stream disconnected in the middle of a turn before completion.
+    ResponseStreamDisconnected {
+        #[serde(rename = "httpStatusCode")]
+        #[ts(rename = "httpStatusCode")]
+        http_status_code: Option<u16>,
+    },
+    /// Reached the retry limit for responses.
+    ResponseTooManyFailedAttempts {
+        #[serde(rename = "httpStatusCode")]
+        #[ts(rename = "httpStatusCode")]
+        http_status_code: Option<u16>,
+    },
+    Other,
+}
+
+impl From<CoreCodexErrorInfo> for CodexErrorInfo {
+    fn from(value: CoreCodexErrorInfo) -> Self {
+        match value {
+            CoreCodexErrorInfo::ContextWindowExceeded => CodexErrorInfo::ContextWindowExceeded,
+            CoreCodexErrorInfo::UsageLimitExceeded => CodexErrorInfo::UsageLimitExceeded,
+            CoreCodexErrorInfo::HttpConnectionFailed { http_status_code } => {
+                CodexErrorInfo::HttpConnectionFailed { http_status_code }
+            }
+            CoreCodexErrorInfo::ResponseStreamConnectionFailed { http_status_code } => {
+                CodexErrorInfo::ResponseStreamConnectionFailed { http_status_code }
+            }
+            CoreCodexErrorInfo::InternalServerError => CodexErrorInfo::InternalServerError,
+            CoreCodexErrorInfo::Unauthorized => CodexErrorInfo::Unauthorized,
+            CoreCodexErrorInfo::BadRequest => CodexErrorInfo::BadRequest,
+            CoreCodexErrorInfo::SandboxError => CodexErrorInfo::SandboxError,
+            CoreCodexErrorInfo::ResponseStreamDisconnected { http_status_code } => {
+                CodexErrorInfo::ResponseStreamDisconnected { http_status_code }
+            }
+            CoreCodexErrorInfo::ResponseTooManyFailedAttempts { http_status_code } => {
+                CodexErrorInfo::ResponseTooManyFailedAttempts { http_status_code }
+            }
+            CoreCodexErrorInfo::Other => CodexErrorInfo::Other,
+        }
+    }
+}
+
 v2_enum_from_core!(
     pub enum AskForApproval from codex_protocol::protocol::AskForApproval {
         UnlessTrusted, OnFailure, OnRequest, Never
@@ -544,11 +609,20 @@ pub struct Turn {
     pub status: TurnStatus,
 }
 
+#[derive(Serialize, Deserialize, Debug, Clone, PartialEq, JsonSchema, TS, Error)]
+#[serde(rename_all = "camelCase")]
+#[ts(export_to = "v2/")]
+#[error("{message}")]
+pub struct TurnError {
+    pub message: String,
+    pub codex_error_code: Option<CodexErrorInfo>,
+}
+
 #[derive(Serialize, Deserialize, Debug, Clone, PartialEq, JsonSchema, TS)]
 #[serde(rename_all = "camelCase")]
 #[ts(export_to = "v2/")]
-pub struct TurnError {
-    pub message: String,
+pub struct ErrorNotification {
+    pub error: TurnError,
 }
 
 #[derive(Serialize, Deserialize, Debug, Clone, PartialEq, JsonSchema, TS)]
@@ -1091,6 +1165,7 @@ mod tests {
     use codex_protocol::items::WebSearchItem;
     use codex_protocol::user_input::UserInput as CoreUserInput;
     use pretty_assertions::assert_eq;
+    use serde_json::json;
     use std::path::PathBuf;
 
     #[test]
@@ -1176,4 +1251,20 @@ mod tests {
             }
         );
     }
+
+    #[test]
+    fn codex_error_code_serializes_http_status_code_in_camel_case() {
+        let value = CodexErrorInfo::ResponseTooManyFailedAttempts {
+            http_status_code: Some(401),
+        };
+
+        assert_eq!(
+            serde_json::to_value(value).unwrap(),
+            json!({
+                "responseTooManyFailedAttempts": {
+                    "httpStatusCode": 401
+                }
+            })
+        );
+    }
 }