CVE-2026-87997: Open WebUI: Any authenticated user can inject chats into another user's folder via chat completions

The chat-completions endpoint reads a folder id out of the request body and saves the newly created chat into that folder without checking that the caller is allowed to write there. Any authenticated user who knows a folder’s id can put a chat of their own into another user’s folder, including a shared folder where they hold read-only access and a folder they have no access to at all. The chat then shows up in that folder for everyone who can read it, under a title and with content the attacker controls. The dedicated chat-creation and chat-move endpoints already enforced this check, the chat-completions path did not.