Support configure skip_special_tokens in openai response api (#32345)

Signed-off-by: Shiyan Deng <dsy842974287@meta.com>

vllm/entrypoints/openai/responses/protocol.py

@@ -171,6 +171,7 @@ class ResponsesRequest(OpenAIBaseModel):
     top_k: int | None = None
     truncation: Literal["auto", "disabled"] | None = "disabled"
     user: str | None = None
+    skip_special_tokens: bool = True
 
     # --8<-- [start:responses-extra-params]
     request_id: str = Field(
@@ -279,6 +280,7 @@ class ResponsesRequest(OpenAIBaseModel):
             structured_outputs=structured_outputs,
             logit_bias=self.logit_bias,
             skip_clone=True,  # Created fresh per request, safe to skip clone
+            skip_special_tokens=self.skip_special_tokens,
         )
 
     def is_include_output_logprobs(self) -> bool: