Moving a folder under one of its own subfolders was accepted. A folder in a parent loop is never a root, so it and everything under it silently disappeared from the sidebar, and there was no way to get it back from the UI.
The move is now rejected with a 400, folders whose parent chain loops are put back at the root on the next folder list, and the folder tree traversals skip ids they have already visited so existing data in that state stays workable.
Chat creation and chat moves each carried their own copy of the same folder_id validation, resolving the folder and checking ownership and shared write access in slightly different ways. Both now call a single has_folder_write_access helper, which the chat-completions creation path uses as well, so ownership, inherited write grants and nonexistent or malformed ids behave identically everywhere a chat folder_id is set. The owner case also costs one query fewer than before.
Classic298