The compute API now exposes folder deletion (cascade: documents and
subfolders; 404 folder_not_found; 409 when descendants are still
queued/processing), so replace the not-supported error with the real call.
The endpoint's own 404 ('Folder not found.') is treated as idempotent
success and drops the cached folder id; a bare route-miss 404 from a server
without the endpoint still raises so the delete can't silently no-op
against old deployments. 409/5xx propagate with the cache intact.
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