Delete a folder and all its contents.

Required permission: WRITER or higher. A READER gets 403; a caller with no role on the knowledge base gets 404.

Cascade delete:

All subfolders and records within are deleted for good, unless "Move Deleted Records to the Trash" (Labs flag ENABLE_SOFT_DELETE) is on: then they all go to the trash and can be brought back with POST /knowledgeBase/record/{folderId}/restore. The response is the same in both cases. Known gap: while in the trash, the folder is still listed in folders by GET /knowledgeBase/{kbId}, although its records answer 404.

Request: kbId and folderId are only checked to be non-empty; any query string or body is ignored.

{
  "success": true,
  "message": "Folder deleted successfully"
}