[Frontend] Allow parsed tool arguments (#28820)

Signed-off-by: Quentin Gallouédec <45557362+qgallouedec@users.noreply.github.com>
Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>

Author: qgallouedec

vllm/entrypoints/chat_utils.py

@@ -1437,7 +1437,8 @@ def _postprocess_messages(messages: list[ConversationMessage]) -> None:
             for item in message["tool_calls"]:
                 # if arguments is None or empty string, set to {}
                 if content := item["function"].get("arguments"):
-                    item["function"]["arguments"] = json.loads(content)
+                    if not isinstance(content, (dict, list)):
+                        item["function"]["arguments"] = json.loads(content)
                 else:
                     item["function"]["arguments"] = {}