[structured output] validate unsupported json features first (#33233)

Signed-off-by: Andy Xie <andy.xning@gmail.com>
Co-authored-by: Chauncey <chaunceyjiang@gmail.com>
Co-authored-by: Russell Bryant <rbryant@redhat.com>

vllm/v1/structured_output/backend_xgrammar.py

@@ -304,17 +304,17 @@ def validate_xgrammar_grammar(sampling_params: SamplingParams) -> None:
         else:
             schema = so_params.json
 
+        if has_xgrammar_unsupported_json_features(schema):
+            raise ValueError(
+                "The provided JSON schema contains features not supported by xgrammar."
+            )
+
         try:
             xgr.Grammar.from_json_schema(schema)
         except Exception as err:
             raise ValueError(
                 f"Failed to transform json schema into a grammar: {err}"
             ) from err
-
-        if has_xgrammar_unsupported_json_features(schema):
-            raise ValueError(
-                "The provided JSON schema contains features not supported by xgrammar."
-            )
         return
 
     if so_params.grammar: