Added new rubocop extensions

[tungleduyxyz]

Related issue: killbill/killbill-admin-ui#345

[Copilot]

Pull request overview

This PR updates RuboCop configuration by adding additional RuboCop extension gems and enabling their cop departments, alongside a few small code tweaks to satisfy/align with the expanded linting ruleset.

Changes:

• Add rubocop-performance, rubocop-rails, and rubocop-thread_safety to the development Gemfile group and require them from .rubocop.yml.
• Enable Rails/Performance/ThreadSafety departments in .rubocop.yml and set TargetRubyVersion.
• Minor refactors: prefer post routes over match ... via: :post, and replace states.values.include? with states.value?.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
Gemfile	Adds RuboCop extension gems to support additional cop departments.
.rubocop.yml	Requires/enables the new RuboCop extensions and configures Ruby target version.
config/routes.rb	Uses explicit post route declarations instead of match ... via: :post.
app/controllers/payment_test/payments_controller.rb	Uses Hash#value? for cleaner/faster value checks.
test/test_helper.rb	Simplifies support file requiring (removes redundant sorting step).

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