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>

api/utils/validation_utils.py

@@ -82,6 +82,8 @@ async def validate_and_parse_json_request(request: Request, validator: type[Base
         2. Extra fields added via `extras` parameter are automatically removed
            from the final output after validation
     """
+    if request.mimetype != "application/json":
+        return None, f"Unsupported content type: Expected application/json, got {request.content_type}"
     try:
         payload = await request.get_json() or {}
     except UnsupportedMediaType: