telemetry: hard-blocked SQL is reported as core_failure error_class unknown #1664

Found during the v0.12.9 end-to-end run (#1647).

A sql_execute refused by the DROP DATABASE / DROP SCHEMA / TRUNCATE block ("... blocked for safety. This cannot be overridden.") is sent as core_failure with error_class: unknown, while a write refused for lack of --allow-write is error_class: permission. Dashboards that count unknown failures will count every deliberate refusal. Suggested: classify the hard block as permission (or a new blocked class).