[typing] fix type (#29964)

Signed-off-by: Andy Xie <andy.xning@gmail.com>

vllm/reasoning/abs_reasoning_parsers.py

@@ -121,7 +121,7 @@ class ReasoningParser:
         self,
         original_tag: str | None,
         tool_server: ToolServer | None,
-    ) -> str:
+    ) -> str | None:
         """
         Instance method that is implemented for preparing the structured tag
         Otherwise, None is returned

vllm/reasoning/gptoss_reasoning_parser.py

@@ -145,7 +145,7 @@ class GptOssReasoningParser(ReasoningParser):
     # This function prepares the structural tag to format reasoning output
     def prepare_structured_tag(
         self, original_tag: str | None, tool_server: ToolServer | None
-    ) -> str:
+    ) -> str | None:
         if original_tag is None:
             if tool_server is None:
                 return json.dumps(no_func_reaonsing_tag)