Raise ActiveRecord::InvalidForeignKey in before_destroy for a blob if attachments exist

The issue #32584 was fixed in #33405 by adding foreign key constraint
to the `active_storage_attachments` table for blobs.
This commit implements fix on app-level in order to ensure that users
can't delete a blob with attachments even if they don't have the foreign key constraint.
See a related discussion in the Campfire:
https://3.basecamp.com/3076981/buckets/24956/chats/12416418@1236718899

Note that, we should backport it to `5-2-stable` too.

Related to #33405

activestorage/app/models/active_storage/blob.rb

@@ -35,6 +35,10 @@ class ActiveStorage::Blob < ActiveRecord::Base
 
   scope :unattached, -> { left_joins(:attachments).where(ActiveStorage::Attachment.table_name => { blob_id: nil }) }
 
+  before_destroy(prepend: true) do
+    raise ActiveRecord::InvalidForeignKey if attachments.exists?
+  end
+
   class << self
     # You can used the signed ID of a blob to refer to it on the client side without fear of tampering.
     # This is particularly helpful for direct uploads where the client-side needs to refer to the blob