feat(batch18): implement group-b server core helpers

dotnet/src/ZB.MOM.NatsNet.Server/NatsServer.Accounts.cs

@@ -638,6 +638,14 @@ public sealed partial class NatsServer
         return (claims, claimJwt, null);
     }
 
+    /// <summary>
+    /// Fetches an account from the resolver, registers it, and returns it.
+    /// Mirrors Go <c>Server.fetchAccount</c>.
+    /// Lock must NOT be held on entry.
+    /// </summary>
+    public (Account? Account, Exception? Error) FetchAccount(string name) =>
+        FetchAccountFromResolver(name);
+
     /// <summary>
     /// Fetches an account from the resolver, registers it, and returns it.
     /// Mirrors Go <c>Server.fetchAccount</c>.