Default to generation_config from model (#12622)

Signed-off-by: Harry Mellor <19981378+hmellor@users.noreply.github.com>

tests/entrypoints/openai/test_serving_chat.py

@@ -38,6 +38,7 @@ class MockModelConfig:
     diff_sampling_param: Optional[dict] = None
     allowed_local_media_path: str = ""
     encoder_config = None
+    generation_config: str = "auto"
 
     def get_diff_sampling_param(self):
         return self.diff_sampling_param or {}