[V1] Set structured output backend to auto by default (#15724)

Signed-off-by: Russell Bryant <rbryant@redhat.com>

vllm/engine/arg_utils.py

@@ -182,7 +182,7 @@ class EngineArgs:
     enable_chunked_prefill: Optional[bool] = None
     disable_chunked_mm_input: bool = False
 
-    guided_decoding_backend: str = 'xgrammar'
+    guided_decoding_backend: str = DecodingConfig.guided_decoding_backend
     logits_processor_pattern: Optional[str] = None
 
     speculative_config: Optional[Dict[str, Any]] = None
@@ -407,13 +407,13 @@ class EngineArgs:
         parser.add_argument(
             '--guided-decoding-backend',
             type=str,
-            default='xgrammar',
+            default=DecodingConfig.guided_decoding_backend,
             help='Which engine will be used for guided decoding'
             ' (JSON schema / regex etc) by default. Currently support '
             'https://github.com/mlc-ai/xgrammar and '
             'https://github.com/guidance-ai/llguidance.'
             'Valid backend values are "xgrammar", "guidance", and "auto". '
-            'With "auto", we will make opinionated choices based on request'
+            'With "auto", we will make opinionated choices based on request '
             'contents and what the backend libraries currently support, so '
             'the behavior is subject to change in each release.')
         parser.add_argument(