Add migration for async validation of routes.namespace_id FK

What does this MR do and why?

This MR adds a post-deployment migration to prepare async validation for the routes.namespace_id foreign key constraint that was introduced in !181772 (merged).

The migration uses prepare_async_foreign_key_validation to schedule the validation of the foreign key fk_679ff8213d on the routes.namespace_id column. This is a required step before the foreign key can be validated synchronously in a future migration.

References

• Closes #535947 (closed)
• Related to !181772 (merged) (where the foreign key was introduced)
• Related to #525273 (closed) (future synchronous validation)

How to set up and validate locally

1. Checkout to this branch
2. Run bin/rails db:migrate
3. Verify the migration runs successfully

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.