Rebuild every attachment before editing a message

The editor keeps an attachment's content as it finds it, so a hand-written
embed with a url but no href reached the editor with its markup unvalidated,
and a mention saved under Trix's editor carried the generic octet-stream
content type the editor doesn't permit and was dropped on save. Rebuilding
each attachment from its attachable renders the hardened preview partial
and restores the mention content type.

test/helpers/rich_text_helper_test.rb

@@ -15,7 +15,37 @@ class RichTextHelperTest < ActionView::TestCase
     assert_equal "https://example.com/image.png", content.at_css(".og-embed__image img")["src"]
   end
 
-  test "editable_body leaves bodies without legacy embeds unchanged" do
+  test "editable_body rebuilds a hand-written embed from its validated details" do
+    content = <<~HTML.squish
+      <actiontext-opengraph-embed data-controller="pwn" data-action="click->pwn#run">
+        <div class="og-embed"><div class="og-embed__title"><a href="/rooms/1">Free cookies</a></div>
+        <div class="og-embed__image"><img src="/rooms/1/avatar" data-action="load->pwn#run"></div></div>
+      </actiontext-opengraph-embed>
+    HTML
+    body = %(<p><action-text-attachment content-type="application/vnd.actiontext.opengraph-embed" url="https://example.com/image.png" content="#{ERB::Util.html_escape(content)}"></action-text-attachment></p>)
+    message = Message.create! room: rooms(:pets), body: body, client_message_id: "0019", creator: users(:jason)
+
+    node = editable_body(message).body.fragment.find_all("action-text-attachment").first
+    rebuilt = Nokogiri::HTML.fragment(node["content"])
+
+    assert_equal "Free cookies", rebuilt.at_css(".og-embed__title").text.strip
+    assert_no_match /rooms\/1/, node["content"]
+    assert_no_match /data-/, node["content"]
+    assert_nil rebuilt.at_css("a")
+    assert_nil rebuilt.at_css("img")
+  end
+
+  test "editable_body restores the content type of a mention edited under Trix" do
+    body = %(<div>Hey <action-text-attachment sgid="#{users(:david).attachable_sgid}" content-type="application/octet-stream"></action-text-attachment></div>)
+    message = Message.create! room: rooms(:pets), body: body, client_message_id: "0020", creator: users(:jason)
+
+    node = editable_body(message).body.fragment.find_all("action-text-attachment").first
+
+    assert_equal "application/vnd.campfire.mention", node["content-type"]
+    assert_match "David", node["content"]
+  end
+
+  test "editable_body leaves bodies without attachments unchanged" do
     message = Message.create! room: rooms(:pets), body: "<p>Plain text</p>", client_message_id: "0018", creator: users(:jason)
 
     assert_equal message.body.body.to_html, editable_body(message).body.to_html