Solve problem when Action Text raises on missing attachables

[jorgemanrubia]

Reverts the previous patch from #2381.

The problem was that I hadn't update Lexxy in the saas version basecamp/lexxy#596

I added a test too. Reproduction script:

# Run with: bin/rails runner tmp/reproduce_missing_attachable.rb
#
# This script reproduces the issue where rendering a rich text field with
# a missing attachable looks for the partial in the wrong namespace
# (e.g., cards/action_text/attachables/_missing_attachable instead of
# action_text/attachables/_missing_attachable)

require "action_controller/test_case"

puts "=" * 60
puts "Testing ActionText attachment rendering with missing attachable"
puts "=" * 60
puts

card = Card.first
puts "Using card: #{card.title} (#{card.id})"

card.update! description: <<~HTML
  <action-text-attachment sgid="gid://fizzy/Card/nonexistent" content-type="application/octet-stream"></action-text-attachment>
HTML

puts "Updated card description with missing attachment HTML"
puts

module Cards
  class TestController < ApplicationController
    def test_render
      view_context.lexxy_rich_textarea_tag(:description, @card.description)
    end
  end
end

controller = Cards::TestController.new
controller.instance_variable_set(:@card, card)

request = ActionDispatch::TestRequest.create
controller.set_request!(request)
controller.set_response!(ActionDispatch::Response.new)

puts "Rendering from Cards:: namespaced controller..."
puts

begin
  controller.test_render
  puts "SUCCESS - No error when rendering missing attachable"
rescue ActionView::MissingTemplate => e
  puts "FAILURE - #{e.message.lines.first}"
  exit 1
end