Add dependency on minitest-mock for MT6 compatibility

[voxik]

This prevents test errors with minitest 6+ such as:

  1) Error:
RailsApiTest#test_best_supported_vendor_when_html5_is_not_supported_returns_html4: NoMethodError: undefined method 'stub' for class Rails::HTML::Sanitizer
    test/rails_api_test.rb:21:in 'RailsApiTest#test_best_supported_vendor_when_html5_is_not_supported_returns_html4'

  2) Error:
RailsApiTest#test_best_supported_vendor_when_html5_is_supported_returns_html5: NoMethodError: undefined method 'stub' for class Rails::HTML::Sanitizer
    test/rails_api_test.rb:29:in 'RailsApiTest#test_best_supported_vendor_when_html5_is_supported_returns_html5'

This is due to minitest-mock being extracted from Minitest 6+:

minitest/minitest@de9aac1

Please note that the condition is in this way, because MT6 restrict upport to Ruby 3.2+ [1, 2]

[voxik]

Please see https://github.com/rails/rails-html-sanitizer/actions/runs/20606018257 for examples of the test failures