FIX: Show large image placeholder for image onebox (#21237)

Large or broken images are removed from oneboxes, but sometimes images
were removed when they were oneboxed too. The reason is that images can
be oneboxed by the AllowlistedGenericOnebox or ImageOnebox and only
AllowlistedGenericOnebox was handled correctly.

spec/lib/cooked_post_processor_spec.rb

@@ -1152,13 +1152,13 @@ RSpec.describe CookedPostProcessor do
 
     it "replaces large image placeholder" do
       SiteSetting.max_image_size_kb = 4096
-      url = "https://image.com/my-avatar"
-      image_url = "https://image.com/avatar.png"
+      url = "https://image.com/avatar.png"
 
-      Oneboxer
-        .stubs(:onebox)
-        .with(url, anything)
-        .returns("<img class='onebox' src='#{image_url}' />")
+      Oneboxer.stubs(:onebox).with(url, anything).returns <<~HTML
+          <a href="#{url}" target="_blank" rel="noopener" class="onebox">
+            <img class='onebox' src='#{url}' />
+          </a>
+        HTML
 
       post = Fabricate(:post, raw: url)