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/apps/__init__.py

@@ -265,7 +265,6 @@ async def not_found(error):
         "message": error_msg,
     }, 404
 
-
 @app.teardown_request
 def _db_close(exception):
     if exception:

api/apps/sdk/chat.py

@@ -176,7 +176,7 @@ async def update(tenant_id, chat_id):
             req["llm_id"] = llm.pop("model_name")
             if req.get("llm_id") is not None:
                 llm_name, llm_factory = TenantLLMService.split_model_name_and_factory(req["llm_id"])
-                model_type = llm.pop("model_type")
+                model_type = llm.get("model_type")
                 model_type = model_type if model_type in ["chat", "image2text"] else "chat"
                 if not TenantLLMService.query(tenant_id=tenant_id, llm_name=llm_name, llm_factory=llm_factory, model_type=model_type):
                     return get_error_data_result(f"`model_name` {req.get('llm_id')} doesn't exist")

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: