refactor: improve error_message test helper robustness

Use Enum.reject for nil field case to explicitly filter errors
without field. Update test to use :is_system_role field since
validation error includes field.

test/mv/authorization/role_test.exs

@@ -53,7 +53,7 @@ defmodule Mv.Authorization.RoleTest do
       assert {:error, %Ash.Error.Invalid{errors: errors}} =
                Authorization.destroy_role(system_role)
 
-      message = error_message(errors, nil)
+      message = error_message(errors, :is_system_role)
       assert message =~ "Cannot delete system role"
     end
 
@@ -84,14 +84,15 @@ defmodule Mv.Authorization.RoleTest do
   end
 
   # Helper function for error evaluation
-  # When field is nil, returns first error message (for errors without specific field)
-  defp error_message(errors, field) when is_nil(field) do
+  # When field is nil, returns first error message for errors without specific field
+  defp error_message(errors, nil) do
     errors
+    |> Enum.reject(fn err -> Map.has_key?(err, :field) end)
     |> Enum.map(&Map.get(&1, :message, ""))
     |> List.first() || ""
   end
 
-  defp error_message(errors, field) do
+  defp error_message(errors, field) when is_atom(field) do
     errors
     |> Enum.filter(fn err -> Map.get(err, :field) == field end)
     |> Enum.map(&Map.get(&1, :message, ""))