Add associations and acts_as_paranoid

[nicklathe]

This PR will be merged after the required migrations are completed. This PR does the following:

• Adds has_many: associations for Users -> LTI User Identities LTI Integrations -> LTI Deployments, as well as dependent: :destroy. This set configures Active Record to delete these child resources when their parent is deleted.
• Adds acts_as_paranoid to the LTI User Identity model, which offers soft deletion of records.
• Adds method to delete attached LTI User Identities when the associated User is purged, as well as corresponding unit test

Links

This PR depends on the following migration PR to be merged first: #54266

Testing story

bundle exec spring testunit ./test/helpers/delete_accounts_helper_test.rb

  144/144: [=========================================================================================] 100% Time: 00:00:27, Time: 00:00:27

Finished in 29.42704s
144 tests, 830 assertions, 0 failures, 0 errors, 0 skips

Deployment strategy

Follow-up work

Privacy

Security

Caching

PR Checklist:

• Tests provide adequate coverage
• Privacy and Security impacts have been assessed
• Code is well-commented
• New features are translatable or updates will not break translations
• Relevant documentation has been added or updated
• User impact is well-understood and desirable
• Pull Request is labeled appropriately
• Follow-up work items (including potential tech debt) are tracked and linked

[carl-codeorg]

LGTM