How to properly assert the return value from Chef library in ChefSpec?

Chef Infra (archive)
1

I have got this library get_full_name to return a full name.

# ../libraries/get_full_name.rb
module MyOrg
  module GetFullName
      def get_full_name(username)
          full_name = "#{node['domain_prefix']}\\#{username}"
      end
  end
end
Chef::Recipe.send(:include, MyOrg::GetFullName)

And a ChefSpec test for the above library.

# ../spec/get_full_name_spec.rb
require 'spec_helper'

Dir['libraries/*.rb'].each { |f| require File.expand_path(f) }

RSpec.configure do |c|
    c.include MyOrg::GetFullName
end

describe MyOrg::GetFullName do
    let(:subject) { Class.new { extend MyOrg::GetFullName } }

    let(:chef_run) do
        ChefSpec::SoloRunner.new(platform: 'windows', version: '2016') do |node|
          node.default['domain_prefix'] = "Example"
        end.converge(described_recipe)
    end

    it 'has valid full name' do
      expect(get_full_name("jenkins")).to be("Example\\jenkins")
    end
end

I have got this error when I ran chefspec

undefined local variable or method `node'

I dont know why it threw that error since I have already specified the node['domain_prefix'] in the ChefSpec.
Could anyone please help me point out what have I missed and is it the correct way to test and assert this type of return value? Would I be able to do the same if I want to test the library in the ChefSpec test for recipes which call this library's function?

Thanks