Discourse doesn't prevent moderators from exporting user Chat DMs

Discourse is an open source discussion platform. Prior to versions 2025.12.2, 2026.1.1, and 2026.2.0, moderators could export user Chat DMs via the CSV export endpoint by exploiting an overly permissive allowlist in `can_export_entity?`. The method allowed moderators to export any entity not explicitly blocked instead of restricting to an explicit allowlist. Versions 2025.12.2, 2026.1.1, and 2026.2.0 patch the issue. No known workarounds are available.