[CI] Make JSON output tests less likely to fail (#17859)

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

tests/v1/entrypoints/llm/test_struct_output_generate.py

@@ -62,6 +62,16 @@ class CarDescription(BaseModel):
     car_type: CarType
 
 
+def _load_json(s: str, backend: str) -> str:
+    if backend != "xgrammar":
+        return json.loads(s)
+
+    # xgrammar specific workarounds
+    # https://github.com/mlc-ai/xgrammar/issues/286
+    s = re.sub(r'[\x00-\x1F\x7F-\xFF]', '', s)
+    return json.loads(s)
+
+
 @pytest.mark.skip_global_cleanup
 @pytest.mark.parametrize(
     "model_name, guided_decoding_backend, tokenizer_mode, speculative_config",
@@ -102,7 +112,7 @@ def test_structured_output(
     #
     sampling_params = SamplingParams(
         temperature=1.0,
-        max_tokens=1000,
+        max_tokens=4096,
         guided_decoding=GuidedDecodingParams(json=sample_json_schema))
     outputs = llm.generate(prompts=[
         (f"Give an example JSON for an employee profile that fits this "
@@ -131,7 +141,7 @@ def test_structured_output(
     #
     sampling_params = SamplingParams(
         temperature=1.0,
-        max_tokens=100,
+        max_tokens=4096,
         n=2,
         guided_decoding=GuidedDecodingParams(json_object=True))
 
@@ -161,7 +171,7 @@ def test_structured_output(
     #
     sampling_params = SamplingParams(
         temperature=1.0,
-        max_tokens=1000,
+        max_tokens=4096,
         guided_decoding=GuidedDecodingParams(json=unsupported_json_schema))
     if guided_decoding_backend.startswith("xgrammar"):
         with pytest.raises(ValueError,
@@ -376,12 +386,13 @@ def test_structured_output(
                 "minLength": min_length
             }
         },
-        "required": ["description"]
+        "required": ["description"],
+        "additionalProperties": False
     }
 
     sampling_params = SamplingParams(
         temperature=1.0,
-        max_tokens=1000,
+        max_tokens=4096,
         guided_decoding=GuidedDecodingParams(json=json_schema))
 
     outputs = llm.generate(
@@ -417,7 +428,8 @@ def test_structured_output(
                     "city": {
                         "type": "string"
                     }
-                }
+                },
+                "additionalProperties": False
             },
             "end": "</function>"
         }],
@@ -426,7 +438,7 @@ def test_structured_output(
 
     sampling_params = SamplingParams(
         temperature=0.0,
-        max_tokens=100,
+        max_tokens=4096,
         guided_decoding=GuidedDecodingParams(
             structural_tag=json.dumps(structural_tag_config)))