Add test for not purging non referenced files

This was probably fixed between Rails 7.1 and Rails c6ef898 (current main)

We encountered this problem on Rails 7.1

actiontext/test/integration/purge_later_test.rb

@@ -0,0 +1,17 @@
+# frozen_string_literal: true
+
+require "test_helper"
+
+class ActionText::PurgeLaterTest < ActiveJob::TestCase
+  test "doesn't delete the files" do
+    blob = create_file_blob(filename: "racecar.jpg", content_type: "image/jpeg")
+    message = Message.create!(content: ActionText::Content.new.append_attachables(blob))
+    assert_equal 1, message.content.body.attachables.size
+    assert_equal 1, message.content.embeds.size
+    message.update!(content: "")
+    perform_enqueued_jobs
+    assert_equal 0, message.content.body.attachables.size
+    assert_equal 1, message.content.embeds.size
+    assert_nothing_raised { blob.download }
+  end
+end