Skip managed settings initialization with pending migrations

What does this MR do and why?

Skip managed settings validation logic when migration is still pending

If we run validations against application_settings table during initialization, we may prevent migrations to run in certain conditions (see: #628124)

References

Fixes #628124

How to set up and validate locally

1. Create a config/managed_settings.yml

   installation:
     managed_by: "GitLab Helm Chart"
   managed_settings:
     sidekiq_timezone_override: "Europe/Lisbon"
   

2. Checkout the master branch, run: git checkout master
3. Rollback a migration that adds a new column to application_settings: rake db:migrate:down:main VERSION=20260818090000
4. Try to run migrations and see it crashes: rake db:migrate
5. Switch back to this MR's branch: git checkout brodock/skip-managed-settings-with-pending-migrations-628124
6. Try running the same migration again: rake db:migrate
7. Should succeed

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