Fix: Add validation for dialog name (#8722)

### What problem does this PR solve?

- Validate dialog name in `dialog_app.py` to ensure it is a non-empty
string and does not exceed 255 bytes in UTF-8 encoding.

### Type of change

- [x] Bug Fix (non-breaking change which fixes an issue)

api/apps/dialog_app.py

@@ -34,6 +34,12 @@ def set_dialog():
     req = request.json
     dialog_id = req.get("dialog_id")
     name = req.get("name", "New Dialog")
+    if not isinstance(name, str):
+        return get_data_error_result(message="Dialog name must be string.")
+    if name.strip() == "":
+        return get_data_error_result(message="Dialog name can't be empty.")
+    if len(name.encode("utf-8")) > 255:
+        return get_data_error_result(message=f"Dialog name length is {len(name)} which is larger than 255")
     description = req.get("description", "A helpful dialog")
     icon = req.get("icon", "")
     top_n = req.get("top_n", 6)