The server exposes only POST /folder and GET /folders (verified against the
full route table and its git history — a DELETE /folder route never
existed; the 0.2.x SDK accordingly shipped create_folder/list_folders but
no delete). The DELETE /folder/{id}/ call could only 404, surfacing as a
misleading 'Not Found' CloudAPIError while leaving the stale folder id in
the cache. Raise a clear not-supported error pointing at the dashboard;
keep the idempotent no-op for missing collections and folderless plans.
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