_resolve_allowed_usernames resolved a DISCORD_ALLOWED_USERS name through member.name,
member.display_name or member.global_name, and the first match took the slot. Any member of a
shared server could set a nickname or display name to an allowlisted username, get the
resolved ID (DMs, tools, approval buttons) and lock the real user out.
Match the unique account username only, as #125034 did for SimpleX display names. An entry
that matches only a display name stays unresolved and gets a connect-time hint. Tests cover
a copied nickname and a copied display name seen before the real user, and the hint; the
Discord docs now say display names and nicknames are never matched.
(cherry picked from commit 252f0bccab39afe94f0f71254e47a8f785ec1fb0)
675bad762a