Use :unprocessable_content for scaffolds with Rack 3.1 or higher

[taketo1113]

Motivation / Background

In Rack v3.1.0, the symbol for HTTP status code 422 was changed from :unprocessable_entity to :unprocessable_content.
Rails supports both :unprocessable_entity and :unprocessable_content, but for scaffolds generated when using Rack 3.1 or higher, it now uses :unprocessable_content.

• Update HTTP status codes and associated symbols rack/rack#2137
• Handle Rack 3.2 unprocessable_{entity,content} rails#53383

Detail

This Pull Request updates controllers generated by scaffolds so that when using Rack v3.1 or higher, the HTTP status code returned in JSON format responses is changed from :unprocessable_entity to :unprocessable_content, aligning with the behavior of Rails scaffolds.

When using Rack v3.1 or higher, the generated scaffold code is updated as follows:

# $ bundle exec rails generate scaffold Post title:string
# app/controllers/posts_controller.rb
...
  # POST /posts or /posts.json
  def create
...
      else
        format.html { render :new, status: :unprocessable_content }
+        format.json { render json: @post.errors, status: :unprocessable_content }
-        format.json { render json: @post.errors, status: :unprocessable_entity }
      end
    end
  end

When using Rack v3.0 or lower, the generated code continues to use :unprocessable_entity as before.

Additional information

Since the jbuilder gem does not depend on actionpack (specifically action_dispatch) in its gemspec,
the choice between :unprocessable_entity and :unprocessable_content is determined by Rack (not by ActionDispatch::Constants::UNPROCESSABLE_CONTENT):

# lib/generators/rails/scaffold_controller_generator.rb
        def status_unprocessable_content
          ::Rack::Utils::SYMBOL_TO_STATUS_CODE.key(422) rescue :unprocessable_content
        end

The behavior of Rack::Utils::SYMBOL_TO_STATUS_CODE.key(422) depends on the Rack version:

Rack::Utils::SYMBOL_TO_STATUS_CODE.key(422)
=> :unprocessable_content # rack 3.1 or higher
=> :unprocessable_entity # rack 3.0 and below

The code for Rack::Utils::SYMBOL_TO_STATUS_CODE can be found here:
https://github.com/rack/rack/blob/v3.2.1/lib/rack/utils.rb#L564-L566
https://github.com/rack/rack/blob/2.0.0/lib/rack/utils.rb#L581-L583