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/index_test.exs

@@ -405,6 +405,27 @@ defmodule MvWeb.UserLive.IndexTest do
     end
   end
 
+  describe "system actor user" do
+    test "does not show system actor user in list", %{conn: conn} do
+      # Ensure system actor exists (e.g. via get_system_actor in conn_with_oidc_user)
+      _system_actor = Mv.Helpers.SystemActor.get_system_actor()
+      system_email = Mv.Helpers.SystemActor.system_user_email()
+
+      conn = conn_with_oidc_user(conn)
+      {:ok, _view, html} = live(conn, "/users")
+
+      refute html =~ system_email,
+             "System actor user (#{system_email}) must not appear in the user list"
+    end
+
+    test "destroying system actor user returns error", %{current_user: current_user} do
+      system_actor = Mv.Helpers.SystemActor.get_system_actor()
+
+      assert {:error, %Ash.Error.Invalid{}} =
+               Ash.destroy(system_actor, domain: Mv.Accounts, actor: current_user)
+    end
+  end
+
   describe "member linking display" do
     test "displays linked member name in user list", %{conn: conn} do
       system_actor = Mv.Helpers.SystemActor.get_system_actor()