[Bugfix] Interleaved thinking keeps compatibility with reasoning_content (#33635)

Signed-off-by: chaunceyjiang <chaunceyjiang@gmail.com>
Co-authored-by: Koushik Dutta <koushd@gmail.com>

vllm/entrypoints/chat_utils.py

@@ -1463,6 +1463,9 @@ def _parse_chat_message_content(
             # Include reasoning if present for interleaved thinking.
             if reasoning is not None:
                 result_msg["reasoning"] = cast(str, reasoning)
+                result_msg["reasoning_content"] = cast(
+                    str, reasoning
+                )  # keep compatibility
         elif role == "tool":
             parsed_msg = _ToolParser(message)
             if "tool_call_id" in parsed_msg: