CI: lint with RuboCop

[heliocosta-dev]

What CI now runs

The existing test matrix (Ruby 3.0–3.3, rspec + gem build) is unchanged. A new lint job runs bundle exec rubocop on Ruby 3.3 for every PR and every push to main.

RuboCop 1.91.0 is pinned exactly as a gemspec development dependency, matching the repo's convention (the Gemfile is just gemspec, and Gemfile.lock stays uncommitted). That lets Dependabot's bundler updates bump it. .rubocop.yml uses TargetRubyVersion: 3.0 and NewCops: enable, and is set to the style the code already follows: double quotes, alias_method, consistent hash and argument indentation, [key, table] hash-rocket alignment, and dev deps in the gemspec. Trailing commas use diff_comma, meaning a comma whenever the closing bracket is on its own line. Run bundle exec rubocop locally (-a for safe fixes).

I picked plain RuboCop over standard. Standard hard-codes {a: 1} hash braces, which would reformat ~370 places in this { a: 1 } codebase, and it can't be configured otherwise.

Code changes

These are safe autocorrections only (rubocop -a, never -A), and every one is syntax-only:

• lib/millionsend/contacts.rb: [:id, :email] becomes %i[id email].
• lib/millionsend/request.rb: adds a trailing comma in the multiline TRANSPORT_ERRORS array.
• spec/resources_spec.rb: 7 string arrays become %w[...], plus 2 trailing commas.

No public method, constant, signature or behavior changes. All 68 specs pass.

Deliberately disabled

• Metrics, LineLength and FetchEnvVar. They would demand refactors or rewrite the intentional ENV[...] nil fallback without catching bugs.
• HashExcept, CollectionCompact and MutableConstant. RuboCop marks their rewrites unsafe, and each changes behavior for some input:
  • except on a HashWithIndifferentAccess also drops string keys that the current reject keeps.
  • compact keeps [key, nil] pairs.
  • Freezing USER_AGENT breaks anyone appending to it, which is currently the only way to customize the UA.

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