Add NOT VALID language ID constraints to language tables

Vasilii Iakliushinrequested to merge
519895-add-not-null-constraints into master

What does this MR do and why?

Adds GitLab.com-only post-deployment migrations that add NOT VALID NOT NULL check constraints to programming_languages.language_id and repository_languages.language_id.

Both columns are still nullable, allowing rows without language IDs to be reintroduced. The constraints are added without blocking deployment, with asynchronous validation left to a separate follow-up. The constraints are not added to db/structure.sql because they are GitLab.com-only.

References

How to set up and validate locally

  1. Review the migration specs for both constraints.
  2. Run the migration specs: bundle exec rspec spec/migrations/20260728130100_add_programming_languages_language_id_not_null_constraint_spec.rb spec/migrations/20260728130102_add_repository_languages_language_id_not_null_constraint_spec.rb

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