Fix RuboCop offenses in generated initializer

[RobertoBarros]

Summary

Updates the generated Devise initializer to comply with the default RuboCop configuration included in newly generated Rails applications. Adds regression assertions for the affected generated lines.

Motivation

Running bin/rubocop after installing Devise in a new Rails 8.1 application reported eight offenses in config/initializers/devise.rb: two string literal offenses and six array spacing offenses.

Verification

• bundle exec ruby -Itest test/generators/install_generator_test.rb — 3 runs, 26 assertions, 0 failures
• bin/test — 878 runs, 2,485 assertions, 0 failures
• Generated a new Rails 8.1 application using this local Devise checkout
• bin/rubocop in the generated application — 24 files inspected, no offenses detected
• Rails boot verified with Rails 8.1.3.1 and Devise 5.0.4
• git diff --check passes