[Bug] Fix attention_backend arg string parsing (#30534)

Signed-off-by: mgoin <mgoin64@gmail.com>

vllm/engine/arg_utils.py

@@ -1649,6 +1649,12 @@ class EngineArgs:
                     "attention_backend and attention_config.backend "
                     "are mutually exclusive"
                 )
+            # Convert string to enum if needed (CLI parsing returns a string)
+            if isinstance(self.attention_backend, str):
+                attention_config.backend = AttentionBackendEnum[
+                    self.attention_backend.upper()
+                ]
+            else:
                 attention_config.backend = self.attention_backend
 
         load_config = self.create_load_config()