```ruby
# frozen_string_literal: true

require 'rails_helper'

RSpec.describe LetsEncrypt::VerificationsController, type: :controller do
  routes { LetsEncrypt::Engine.routes }
  class OtherModel < LetsEncrypt::Certificate
  end
  before(:each) do
    LetsEncrypt.config.certificate_model = 'OtherModel'
  end

  it 'returns 404 status when no valid verification path found' do
    open(:get, :show, verification_path: :invalid_path)
    expect(response.status).to eq(404)
  end

  context 'has certificate' do
    let!(:certificate) do
      LetsEncrypt.certificate_model.create(
        domain: 'example.com',
        verification_path: '.well-known/acme-challenge/valid_path',
        verification_string: 'verification'
      )
    end

    it 'returns verification string when found verification path' do
      open(:get, :show, verification_path: 'valid_path')
      expect(response.status).to eq(200)
      expect(response.body).to eq(certificate.verification_string)
    end
  end
end
```

Add test to check controller will work properly with default model and customize model
user
```ruby
# frozen_string_literal: true

require 'rails_helper'

RSpec.describe LetsEncrypt::VerificationsController, type: :controller do
  routes { LetsEncrypt::Engine.routes }
  class OtherModel < LetsEncrypt::Certificate
  end

  it 'returns 404 status when no valid verification path found' do
    open(:get, :show, verification_path: :invalid_path)
    expect(response.status).to eq(404)
  end


  describe 'has certificate' do
   let!(:certificate) do
      LetsEncrypt.certificate_model.create(
        domain: 'example.com',
        verification_path: '.well-known/acme-challenge/valid_path',
        verification_string: 'verification'
      )
    end
    context 'with default model' do
      it 'returns verification string when found verification path' do
        open(:get, :show, verification_path: 'valid_path')
        expect(response.status).to eq(200)
        expect(response.body).to eq(certificate.verification_string)
      end
    end

    context 'with customize model' do
      LetsEncrypt.config.certificate_model = 'OtherModel'
      it 'set the certificate_model to customize model' do
        expect(LetsEncrypt.certificate_model).to eq('OtherModel'.constantize)
      end

      it 'returns verification string when found verification path' do
        open(:get, :show, verification_path: 'valid_path')
        expect(response.status).to eq(200)
        expect(response.body).to eq(certificate.verification_string)
      end
    end
  end
end
```
assistant