Add tests for system actor protection and hiding

Index: system actor not in list, destroy returns Ash.Error.Invalid. Show/Form:
redirect to /users when viewing or editing system actor user.

test/mv_web/user_live/form_test.exs

@@ -420,4 +420,14 @@ defmodule MvWeb.UserLive.FormTest do
       assert is_nil(updated_user.member)
     end
   end
+
+  describe "system actor user" do
+    test "redirects to user list when editing system actor user", %{conn: conn} do
+      system_actor = Mv.Helpers.SystemActor.get_system_actor()
+      conn = conn_with_oidc_user(conn, %{email: "admin@example.com"})
+
+      assert {:error, {:live_redirect, %{to: "/users"}}} =
+               live(conn, "/users/#{system_actor.id}/edit")
+    end
+  end
 end