Rails/UniqueValidationWithoutIndex should not fail when a unique constraint is set

[RubenSmit]

In Rails 7.1 support was added for PostgreSQL unique constraints (rails/rails#46192). This makes it possible to defer checking for constraints inside transactions. This means uniqueness checking can be moved from the index to an unique constraint with add_unique_constraint and this is added to the schema.
Currently rubocop checks the schema for a uniqueness index on the field when a unique validation is set. But it should also accept a unique constraint.

---

Expected behavior

The unique validation without index cop should not fail when a unique constraint is set on the database column.

Actual behavior

Rails/UniqueValidationWithoutIndex: Uniqueness validation should have a unique index on the database column.

Steps to reproduce the problem

In Rails 7.1 or newer with a PostgreSQL database, create an index for a number field of a model with a deferrable uniqueness constraint.

class DeferModelFieldUniqueness < ActiveRecord::Migration
  def change
    # Note that we add an index without enforcing uniqueness
    add_index :model, :field, name: 'index_models_on_field'

    # Ensure the field is unique. Deferrable is set to true to enable changing the field in bulk inside a transaction
    add_unique_constraint :model,
                          :field,
                          deferrable: :deferred,
                          name: 'unique_model_field'
  end
end

Add a validation to the model:

  validates :field, uniqueness: true

RuboCop version

$ [bundle exec] rubocop -V
1.69.1 (using Parser 3.3.6.0, rubocop-ast 1.36.2, analyzing as Ruby 3.3, running on ruby 3.3.5) [x86_64-linux]
  - rubocop-capybara 2.21.0
  - rubocop-factory_bot 2.26.1
  - rubocop-performance 1.20.2
  - rubocop-rails 2.26.2
  - rubocop-rspec 2.31.0
  - rubocop-rspec_rails 2.29.1