Upgrade Rails to 7.1.2 and fix signed_id error

[littleforest]

Background

When upgrading a project that uses this gem to Rails 7.1, an error would occur when a new record that used the Field::ActiveStorage field in the Administrate dashboard failed to save, resulting in an ActionView::Template::Error: Cannot get a signed_id for a new record error. This error occurred since the ActiveStorage::Attachment record was not persisted to the database. In rails/rails#47027, a change was made to raise an error if the record calling signed_id was not persisted.

Changes

This upgrades the test app to Rails 7.1.2, and adds more integration test coverage for the create method. To more easily test the case where a record failed to save, a new required title field was added to the Post model. To fix the underlying issue, the code that is responsible for displaying the image preview and a link to the image is wrapped in a conditional that checks for persistence of the ActiveStorage::Attachment record.

closes #134