Add workaround for shared field_names in pydantic model class (#13925)

Signed-off-by: Max de Bayser <mbayser@br.ibm.com>

Author: maxdebayser

vllm/entrypoints/openai/protocol.py

@@ -1238,6 +1238,9 @@ class ChatCompletionLogProb(OpenAIBaseModel):
 
 
 class ChatCompletionLogProbsContent(ChatCompletionLogProb):
+    # Workaround: redefine fields name cache so that it's not
+    # shared with the super class.
+    field_names: ClassVar[Optional[set[str]]] = None
     top_logprobs: list[ChatCompletionLogProb] = Field(default_factory=list)