temporarily relax ResponsesAPIResponse parsing to support custom backends (#15362)

- Replace `ResponsesAPIResponse(**raw_response_json)` with
  `ResponsesAPIResponse.model_construct(**raw_response_json)`
  to bypass strict Pydantic validation.
- Allows Response API to work with self-hosted or non-strict models
  (e.g., vLLM) without raising literal/type errors.
- This is a temporary, low-risk relaxation to improve compatibility
  with more backend implementations.

litellm/llms/openai/responses/transformation.py

@@ -169,7 +169,7 @@ class OpenAIResponsesAPIConfig(BaseResponsesAPIConfig):
             raise OpenAIError(
                 message=raw_response.text, status_code=raw_response.status_code
             )
-        return ResponsesAPIResponse(**raw_response_json)
+        return ResponsesAPIResponse.model_construct(**raw_response_json)
 
     def validate_environment(
         self, headers: dict, model: str, litellm_params: Optional[GenericLiteLLMParams]