Fixes failing ActionText::ContentTest test cases

The test cases where failing because the test
`test_converts_Trix-formatted_attachments_with_custom_tag_name` set a
custom tag_name to `arbitrary-tag`. This test would also set the
ActionText::AttachmentGallery::ATTACHMENT_SELECTOR private constant
to `arbitrary-tag`.

Other test cases had proper ActionText::Attachment.tag_name set to
`action-text-attachment` but the constant once defined would not
reset.

This PR attempts to fix the issue by converting the
ActionText::AttachmentGallery::{ATTACHMENT_}SELECTOR to class methods

Fixes #41782

actiontext/lib/action_text/attachment_gallery.rb

@@ -4,6 +4,9 @@ module ActionText
   class AttachmentGallery
     include ActiveModel::Model
 
+    TAG_NAME = "div"
+    private_constant :TAG_NAME
+
     class << self
       def fragment_by_canonicalizing_attachment_galleries(content)
         fragment_by_replacing_attachment_gallery_nodes(content) do |node|
@@ -20,12 +23,12 @@ module ActionText
       end
 
       def find_attachment_gallery_nodes(content)
-        Fragment.wrap(content).find_all(SELECTOR).select do |node|
+        Fragment.wrap(content).find_all(selector).select do |node|
           node.children.all? do |child|
             if child.text?
               /\A(\n|\ )*\z/.match?(child.text)
             else
-              child.matches? ATTACHMENT_SELECTOR
+              child.matches? attachment_selector
             end
           end
         end
@@ -34,6 +37,14 @@ module ActionText
       def from_node(node)
         new(node)
       end
+
+      def attachment_selector
+        "#{ActionText::Attachment.tag_name}[presentation=gallery]"
+      end
+
+      def selector
+        "#{TAG_NAME}:has(#{attachment_selector} + #{attachment_selector})"
+      end
     end
 
     attr_reader :node
@@ -43,7 +54,7 @@ module ActionText
     end
 
     def attachments
-      @attachments ||= node.css(ATTACHMENT_SELECTOR).map do |node|
+      @attachments ||= node.css(ActionText::AttachmentGallery.attachment_selector).map do |node|
         ActionText::Attachment.from_node(node).with_full_attributes
       end
     end
@@ -55,11 +66,5 @@ module ActionText
     def inspect
       "#<#{self.class.name} size=#{size.inspect}>"
     end
-
-    TAG_NAME = "div"
-    ATTACHMENT_SELECTOR = "#{ActionText::Attachment.tag_name}[presentation=gallery]"
-    SELECTOR = "#{TAG_NAME}:has(#{ATTACHMENT_SELECTOR} + #{ATTACHMENT_SELECTOR})"
-
-    private_constant :TAG_NAME, :ATTACHMENT_SELECTOR, :SELECTOR
   end
 end