Qualify Struct reference to avoid collisions with possible cop namespace

[technicalpickles]

We were trying to add a custom cop called Struct/RequiresTypedStructHelper, but ran into a problem where we got an error like:

undefined method `new' for RuboCop::Cop::Struct:Module
/Users/technicalpickles/.rbenv/versions/3.2.2/lib/ruby/gems/3.2.0/gems/rubocop-rails-2.21.1/lib/rubocop/cop/mixin/index_method.rb:105:in `module:IndexMethod'
/Users/technicalpickles/.rbenv/versions/3.2.2/lib/ruby/gems/3.2.0/gems/rubocop-rails-2.21.1/lib/rubocop/cop/mixin/index_method.rb:6:in `module:Cop'
/Users/technicalpickles/.rbenv/versions/3.2.2/lib/ruby/gems/3.2.0/gems/rubocop-rails-2.21.1/lib/rubocop/cop/mixin/index_method.rb:4:in `module:RuboCop'
/Users/technicalpickles/.rbenv/versions/3.2.2/lib/ruby/gems/3.2.0/gems/rubocop-rails-2.21.1/lib/rubocop/cop/mixin/index_method.rb:3:in `top (required)'
/Users/technicalpickles/.rbenv/versions/3.2.2/lib/ruby/gems/3.2.0/gems/rubocop-rails-2.21.1/lib/rubocop/cop/rails_cops.rb:7:in `require_relative'
/Users/technicalpickles/.rbenv/versions/3.2.2/lib/ruby/gems/3.2.0/gems/rubocop-rails-2.21.1/lib/rubocop/cop/rails_cops.rb:7:in `top (required)'
/Users/technicalpickles/.rbenv/versions/3.2.2/lib/ruby/gems/3.2.0/gems/rubocop-rails-2.21.1/lib/rubocop-rails.rb:16:in `require_relative'

It turned out this module has method that were calling Struct.new, but Ruby constant lookup ended up using our Rubocop::Cop::Struct, instead of the actual Struct class.

We were able to work around it by changing the constant name, but Struct is always going to be part of the stdlib so a possible name collision.

Replace this text with a summary of the changes in your PR.
The more detailed you are, the better.

---

Before submitting the PR make sure the following are checked:

• The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Ran bundle exec rake default. It executes all tests and runs RuboCop on its own code.
• Added an entry (file) to the changelog folder named {change_type}_{change_description}.md if the new code introduces user-observable changes. See changelog entry format for details.
• If this is a new cop, consider making a corresponding update to the Rails Style Guide.

[technicalpickles]

Added tests.

The main way I can think to test this is to have a new cop that uses the Struct namespace, and is setup to run. I wasn't able to figure out how to set that up just for testing though.

Added an entry (file) to the changelog folder named {change_type}_{change_description}.md if the new code introduces user-observable changes. See changelog entry format for details.

I don't think this is user observable, unless you tried to use make a cop with the Struct namespace.

[koic]

Thanks!