This would fail if `spec/models/concerns/bulk_insert_safe_spec.rb` and
`spec/lib/gitlab/doctor/secrets_spec.rb` were run in order.

The test failed because `bulk_insert_items` is a temporary table
inserted by the test and dropped afterwards. However, the test also
defines a `BulkInsertItem` class, which the GitLab Doctor secrets task
uses to identify encrypted columns.

To fix this, we change `BulkInsertItem` to inherit from
`ActiveRecord::Base` instead of `ApplicationRecord`. We also make sure
the first spec resets the model cache after dropping the table for
completeness.

Closes https://gitlab.com/gitlab-org/gitlab/-/issues/221784