feat: execute full-repo remaining parity closure plan

src/NATS.Server/Auth/ProxyAuthenticator.cs

@@ -0,0 +1,27 @@
+namespace NATS.Server.Auth;
+
+public sealed class ProxyAuthenticator(ProxyAuthOptions options) : IAuthenticator
+{
+    public AuthResult? Authenticate(ClientAuthContext context)
+    {
+        if (!options.Enabled)
+            return null;
+
+        var username = context.Opts.Username;
+        if (string.IsNullOrEmpty(username))
+            return null;
+
+        if (!username.StartsWith(options.UsernamePrefix, StringComparison.Ordinal))
+            return null;
+
+        var identity = username[options.UsernamePrefix.Length..];
+        if (identity.Length == 0)
+            return null;
+
+        return new AuthResult
+        {
+            Identity = identity,
+            AccountName = options.Account,
+        };
+    }
+}