Codegen'd Ruby with generate_types: true doesn't extend T::Sig

[davebenvenuti]

evaling generated Ruby with generate_types: true results in an undefined method error:

https://github.com/Shopify/protoboeuf/blob/f537118067e6f2b1472fb9beb206d110d1ecc96a/test/fixtures/typed_test.correct.rb

irb(main):002* Module.new do
irb(main):003*   class_eval File.read("test/fixtures/typed_test.correct.rb")
irb(main):004> end
(eval at (irb):3):11:in `<module:TestEnum>': undefined method `sig' for module #<Module:0x000000011de4afc0>::TestEnum (NoMethodError)

  sig { params(val: Integer).returns(Symbol) }
  ^^^
        from (eval at (irb):3):6:in `block in <top (required)>'
        from (irb):3:in `class_eval'
        from (irb):3:in `block in <top (required)>'
        from (irb):2:in `initialize'
        from (irb):2:in `new'
        from (irb):2:in `<main>'
        from <internal:kernel>:187:in `loop'
        from /Users/dave/.gem/ruby/3.3.4/gems/irb-1.14.3/exe/irb:9:in `<top (required)>'
        from /Users/dave/.gem/ruby/3.3.4/bin/irb:25:in `load'
        from /Users/dave/.gem/ruby/3.3.4/bin/irb:25:in `<main>'
irb(main):005>

I understand in some environments people extend T::Sig in Object so this is unnecessary, but we can't guarantee that to be the case. This evaled code should work in a vanilla Ruby environment.

Tasks

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1. Generated code should extend T::Sig
2. Improve CodegenTest#test_generate_types to use T.signature_for_method and properly assert that the generated types are what we expect by using class_eval or module_eval on the generated code