ActiveStorage::Attachment#signed_id must return a signed *blob* ID

Fixes that calling e.g. `url_for @user.avatar` would return an incorrect URL resulting in a 404. The Active Storage URL helper ends up calling ActiveStorage::Attachment#signed_id, which previously delegated to ActiveStorage::Blob#signed_id but changed to return a signed *attachment* ID in 1a3dc42c.

The Active Storage controllers pass signed IDs to ActiveStorage::Blob.find_signed, so we need signed blob IDs.

activestorage/app/models/active_storage/attachment.rb

@@ -14,6 +14,7 @@ class ActiveStorage::Attachment < ActiveRecord::Base
   belongs_to :blob, class_name: "ActiveStorage::Blob"
 
   delegate_missing_to :blob
+  delegate :signed_id, to: :blob
 
   after_create_commit :mirror_blob_later, :analyze_blob_later, :identify_blob
   after_destroy_commit :purge_dependent_blob_later

activestorage/test/models/attachment_test.rb

@@ -49,4 +49,12 @@ class ActiveStorage::AttachmentTest < ActiveSupport::TestCase
       assert ActiveStorage::Blob.service.mirrors.second.exist?(blob.key)
     end
   end
+
+  test "getting a signed blob ID from an attachment" do
+    blob = create_blob
+    @user.avatar.attach(blob)
+
+    signed_id = @user.avatar.signed_id
+    assert_equal blob, ActiveStorage::Blob.find_signed(signed_id)
+  end
 end