fix(o-series): generalize is_model_o_series_model to match any o+digit prefix

Replace the hardcoded startswith(("o1", "o3", "o4")) check with a broader
o+digit pattern, ensuring future o-series models (e.g. o2, o5) are
automatically recognized once added to open_ai_chat_completion_models.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

litellm/llms/openai/chat/o_series_transformation.py

@@ -131,7 +131,10 @@ class OpenAIOSeriesConfig(OpenAIGPTConfig):
 
     def is_model_o_series_model(self, model: str) -> bool:
         model = model.split("/")[-1]  # could be "openai/o3" or "o3"
-        return model.startswith(("o1", "o3", "o4")) and model in litellm.open_ai_chat_completion_models
+        return (
+            len(model) > 1 and model[0] == "o" and model[1].isdigit()
+            and model in litellm.open_ai_chat_completion_models
+        )
 
     @overload
     def _transform_messages(