activerecord: Instance self type for the :default option argument in belongs_to

[felixefelip]

In Active Record belongs_to option :default, the self type in block is the instance type instead the singleton type. Doc: https://apidock.com/rails/ActiveRecord/Associations/ClassMethods/belongs_to#:~:text=belongs%5Fto%20%3Aaccount%2C%20default%3A%20%2D%3E%20%7B%20company%2Eaccount%20%7D

In applications like Fizzy, it's a very common use.

Without this fix, we have this output error:

activerecord-8.0.rb:37:36: [error] Type `singleton(::Test::Article)` does not have method `default_user`
│ Diagnostic ID: Ruby::NoMethod
│
└     belongs_to :user, default: -> { default_user }
                                      ~~~~~~~~~~~~

Detected 1 problem from 1 file

[felixefelip]

For the Active Record 6, I couldn't find a test setup, so I didn't add a test to this change. But if it's necessary, I'd be happy to add tests for this version as well.

[github-actions]

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