fix(errors): properly assign APIError.body (#949)

Co-authored-by: Hao Cen <hao_cen@Haos-MacBook-Pro.local>

src/openai/_exceptions.py

@@ -48,6 +48,7 @@ def __init__(self, message: str, request: httpx.Request, *, body: object | None)
         super().__init__(message)
         self.request = request
         self.message = message
+        self.body = body
 
         if is_dict(body):
             self.code = cast(Any, body.get("code"))