Add test coverage for interface! rejecting classes

[mliem2k]

Summary

interface! already raises TypeError when called on a Class:

class MyClass
  interface! # already raises TypeError
end

This also already covers the class << self case from the issue, since a module's singleton class is itself an instance of Class:

module MyInterface
  class << self
    interface! # already raises TypeError, since MyInterface.singleton_class is a Class
  end
end

This existing guard (in TypeToolkit.make_interface!) wasn't covered by any spec, so it was easy to lose track of. This PR adds specs for both raising cases, plus a sanity check that interface! still works normally on a plain module.

Fixes #4

Test plan

• bundle exec rake test — 92 runs, 0 failures
• bundle exec rake rubocop — no offenses
• bundle exec rake typecheck — no errors

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