feat: convert Codex Chat error responses to Responses envelope

The Codex Chat-to-Responses bridge already rewrites successful upstream
responses to the Responses shape, but the error branch in
handle_codex_chat_to_responses_transform passed Chat-shaped error bodies
through untouched (MiniMax base_resp, raw OpenAI Chat error, text/plain
"Unauthorized" pages, etc.), leaving Codex clients unable to recognize the
error.

Add chat_error_to_response_error in transform_codex_chat to regularize all
upstream error shapes into the standard {error: {message, type, code, param}}
envelope, then wire it through a new handle_codex_chat_error_response that
preserves the original HTTP status code. Non-JSON error bodies (HTML, plain
text) are wrapped as Value::String and truncated to 1KB at a UTF-8 char
boundary to keep diagnostic context without flooding the response.

Also fix a pre-existing append-vs-insert pitfall in three rebuilt-body
branches (Claude transform, Codex Chat normal, Codex Chat error): http
Builder::header is append, so leaking the upstream Content-Type produced
two Content-Type headers when the rewritten body was JSON. Remove the
upstream value before writing application/json.

src-tauri/src/proxy/providers/transform_codex_chat.rs

@@ -870,6 +870,80 @@ pub(crate) fn response_status_from_finish_reason(finish_reason: Option<&str>) ->
     }
 }
 
+/// 把 Chat Completions 上游的错误体规整成 OpenAI Responses API 风格的错误对象。
+///
+/// 兼容三类输入：
+/// 1. 标准 OpenAI 形式 `{"error": {"message": "...", "type": "...", "code": ...}}`
+/// 2. MiniMax 等非标形式（如 `{"base_resp": {"status_code": 2013, "status_msg": "..."}}`）
+/// 3. 顶层只有 `message` / `detail` / 裸字符串的最小错误
+///
+/// 输出统一为 `{"error": {"message", "type", "code", "param"}}`，与 OpenAI Responses
+/// API 错误响应一致；Codex 客户端的错误处理只识别这个形状。
+pub fn chat_error_to_response_error(body: Option<&Value>) -> Value {
+    let Some(value) = body else {
+        return json!({
+            "error": {
+                "message": "Upstream returned an empty error response",
+                "type": "upstream_error",
+                "code": serde_json::Value::Null,
+                "param": serde_json::Value::Null,
+            }
+        });
+    };
+
+    if let Some(text) = value.as_str() {
+        return json!({
+            "error": {
+                "message": text,
+                "type": "upstream_error",
+                "code": serde_json::Value::Null,
+                "param": serde_json::Value::Null,
+            }
+        });
+    }
+
+    let source = value.get("error").unwrap_or(value);
+
+    let message = source
+        .get("message")
+        .or_else(|| source.get("detail"))
+        .or_else(|| source.get("status_msg"))
+        .or_else(|| source.pointer("/base_resp/status_msg"))
+        .and_then(|v| v.as_str())
+        .map(ToString::to_string)
+        .or_else(|| source.as_str().map(ToString::to_string))
+        .unwrap_or_else(|| {
+            // 没法从字段提取出文本，就把整个 JSON 序列化回去，方便用户排查。
+            serde_json::to_string(source).unwrap_or_else(|_| "Upstream error".to_string())
+        });
+
+    let error_type = source
+        .get("type")
+        .and_then(|v| v.as_str())
+        .map(ToString::to_string)
+        .unwrap_or_else(|| "upstream_error".to_string());
+
+    let code = source
+        .get("code")
+        .cloned()
+        .or_else(|| source.pointer("/base_resp/status_code").cloned())
+        .unwrap_or(serde_json::Value::Null);
+
+    let param = source
+        .get("param")
+        .cloned()
+        .unwrap_or(serde_json::Value::Null);
+
+    json!({
+        "error": {
+            "message": message,
+            "type": error_type,
+            "code": code,
+            "param": param,
+        }
+    })
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
@@ -1458,4 +1532,80 @@ mod tests {
         assert_eq!(result["status"], "incomplete");
         assert_eq!(result["incomplete_details"]["reason"], "max_output_tokens");
     }
+
+    #[test]
+    fn chat_error_to_response_error_normalizes_standard_openai_shape() {
+        let input = json!({
+            "error": {
+                "message": "Invalid API key",
+                "type": "invalid_request_error",
+                "code": "invalid_api_key",
+                "param": "api_key"
+            }
+        });
+
+        let result = chat_error_to_response_error(Some(&input));
+
+        assert_eq!(result["error"]["message"], "Invalid API key");
+        assert_eq!(result["error"]["type"], "invalid_request_error");
+        assert_eq!(result["error"]["code"], "invalid_api_key");
+        assert_eq!(result["error"]["param"], "api_key");
+    }
+
+    #[test]
+    fn chat_error_to_response_error_normalizes_minimax_base_resp() {
+        // MiniMax 把错误塞在 base_resp 里，code 是数字而不是字符串
+        let input = json!({
+            "base_resp": {
+                "status_code": 2013,
+                "status_msg": "invalid params, chat content has invalid message role: system"
+            }
+        });
+
+        let result = chat_error_to_response_error(Some(&input));
+
+        assert_eq!(
+            result["error"]["message"],
+            "invalid params, chat content has invalid message role: system"
+        );
+        assert_eq!(result["error"]["code"], 2013);
+        // type 没有显式给出，应该回落到 upstream_error
+        assert_eq!(result["error"]["type"], "upstream_error");
+    }
+
+    #[test]
+    fn chat_error_to_response_error_handles_plain_text_body() {
+        let input = json!("Upstream timeout");
+
+        let result = chat_error_to_response_error(Some(&input));
+
+        assert_eq!(result["error"]["message"], "Upstream timeout");
+        assert_eq!(result["error"]["type"], "upstream_error");
+        assert!(result["error"]["code"].is_null());
+        assert!(result["error"]["param"].is_null());
+    }
+
+    #[test]
+    fn chat_error_to_response_error_handles_missing_body() {
+        let result = chat_error_to_response_error(None);
+
+        assert_eq!(
+            result["error"]["message"],
+            "Upstream returned an empty error response"
+        );
+        assert_eq!(result["error"]["type"], "upstream_error");
+    }
+
+    #[test]
+    fn chat_error_to_response_error_falls_back_to_detail_field() {
+        // 部分中转把错误塞在顶层 detail 字段（OpenAI 兼容层常见）
+        let input = json!({
+            "detail": "rate limit exceeded"
+        });
+
+        let result = chat_error_to_response_error(Some(&input));
+
+        assert_eq!(result["error"]["message"], "rate limit exceeded");
+        assert_eq!(result["error"]["type"], "upstream_error");
+    }
 }