Add test exercising image_processing variant generation

Guards against regressions from image_processing version upgrades
(e.g. 2.x drops mini_magick and changes the vips pipeline API).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: nbudin

test/models/active_storage_attachment_test.rb

@@ -0,0 +1,24 @@
+# frozen_string_literal: true
+
+require "test_helper"
+
+class ActiveStorageAttachmentTest < ActiveSupport::TestCase
+  let(:convention) { create(:convention) }
+  let(:event) { create(:event, convention:) }
+
+  it "can generate a resized variant of an attached image" do
+    event.images.attach(
+      io: Rails.root.join("test/files/war_bond.png").open,
+      filename: "war_bond.png",
+      content_type: "image/png"
+    )
+
+    attachment = event.images.first
+    assert attachment.representable?, "Expected PNG attachment to be representable"
+
+    # Exercises image_processing - calls into the vips pipeline via resize_to_limit.
+    # This is the same operation used by ActiveStorageAttachmentType#resized_url.
+    variant = attachment.representation(resize_to_limit: [100, 100]).processed
+    assert variant.present?
+  end
+end