Fix action text extra trix content wrapper

[alexandreruban]

Closes #42315

Summary

When copying an ActionText::RichText between two models, an extra <div class="trix-content">...</div> was added as described in the issue. This comes from the fact that an ActionText::RichText is serialized in a ActionText::Content.

# app/models/action_text/rich_text.rb

module ActionText
  class RichText < Record
    serialize :body, ActionText::Content
  end
end

When the serialization happens in the ActionText::Content.dump method, we didn't have a case to handle serialization of ActionText::RichText, but there is no need to serialize again. We can simply retrieve the HTML body of the ActionText::Content associatied with the ActionText::RichText.