Fix /model_info serialization when a config value is a class (#39237)

Co-authored-by: Lianmin Zheng <lianminzheng@gmail.com>

test/registered/unit/entrypoints/test_server_info.py

@@ -40,6 +40,10 @@ from sglang.test.test_utils import CustomTestCase
 register_cpu_ci(est_time=13, suite="base-a-test-cpu")
 
 
+class _CustomModelLoader:
+    pass
+
+
 def _stub_tokenizer_manager(
     server_args: ServerArgs, get_internal_state=None
 ) -> TokenizerManager:
@@ -57,6 +61,45 @@ def _stub_tokenizer_manager(
     return tokenizer_manager
 
 
+class TestModelInfoSerialization(CustomTestCase):
+    def test_model_info_serializes_custom_loader_class(self):
+        server_args = ServerArgs(model_path="dummy")
+        values = {
+            "weight_version": None,
+            "load_format": _CustomModelLoader,
+            "reasoning_parser": None,
+            "tool_call_parser": None,
+        }
+        tokenizer_manager = SimpleNamespace(
+            model_config=SimpleNamespace(
+                is_image_understandable_model=False,
+                is_audio_understandable_model=False,
+                hf_config=SimpleNamespace(
+                    model_type="test", architectures=["TestModel"]
+                ),
+                embedding_model_spec=None,
+            ),
+            model_path="dummy",
+            served_model_name="dummy",
+            server_args=server_args,
+            is_generation=True,
+            config_value=values.__getitem__,
+        )
+        prior_state = http_server.get_global_state()
+        http_server.set_global_state(
+            SimpleNamespace(tokenizer_manager=tokenizer_manager)
+        )
+        publish(server_args, role="tokenizer")
+        try:
+            payload = asyncio.run(http_server.model_info())
+        finally:
+            http_server._global_state = prior_state
+            reset_context()
+
+        self.assertEqual(payload["load_format"], f"{__name__}._CustomModelLoader")
+        json.dumps(payload)
+
+
 def _call_server_info_with(
     server_args: ServerArgs,
     internal_states: list[dict] | None = None,