[Bugfix] Fix AttributeError when passing StructuredOutputsParams to CompletionRequest (#35237)

Signed-off-by: Patrick Simianer <patrick@lilt.com>

vllm/entrypoints/openai/chat_completion/protocol.py

@@ -555,8 +555,16 @@ class ChatCompletionRequest(OpenAIBaseModel):
             return data
 
         structured_outputs_kwargs = data["structured_outputs"]
+        # structured_outputs may arrive as a dict (from JSON/raw kwargs) or
+        # as a StructuredOutputsParams dataclass instance.
+        is_dataclass = isinstance(structured_outputs_kwargs, StructuredOutputsParams)
         count = sum(
-            structured_outputs_kwargs.get(k) is not None
+            (
+                getattr(structured_outputs_kwargs, k, None)
+                if is_dataclass
+                else structured_outputs_kwargs.get(k)
+            )
+            is not None
             for k in ("json", "regex", "choice")
         )
         # you can only use one kind of constraints for structured outputs