Align p3 HTTP/SDK tests with current backend behavior (#12563)

### What problem does this PR solve?

Updates pre-existing HTTP API and SDK tests to align with current
backend behavior (validation errors, 404s, and schema defaults). This
ensures p3 regression coverage is accurate without changing production
code.

### Type of change

- [x] Other (please describe): align p3 HTTP/SDK tests with current
backend behavior

---------

Co-authored-by: Liu An <asiro@qq.com>

test/testcases/test_http_api/test_router_errors.py

@@ -0,0 +1,29 @@
+#
+#  Copyright 2025 The InfiniFlow Authors. All Rights Reserved.
+#
+#  Licensed under the Apache License, Version 2.0 (the "License");
+#  you may not use this file except in compliance with the License.
+#  You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+#  Unless required by applicable law or agreed to in writing, software
+#  distributed under the License is distributed on an "AS IS" BASIS,
+#  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+#  See the License for the specific language governing permissions and
+#  limitations under the License.
+#
+import pytest
+import requests
+
+from configs import HOST_ADDRESS, VERSION
+
+
+@pytest.mark.p3
+def test_route_not_found_returns_json():
+    url = f"{HOST_ADDRESS}/api/{VERSION}/__missing_route__"
+    res = requests.get(url)
+    assert res.status_code == 404
+    payload = res.json()
+    assert payload["error"] == "Not Found"
+    assert f"/api/{VERSION}/__missing_route__" in payload["message"]