Run the full test suite across supported Rubies and platforms

[tas50]

Summary

Before this change, CI skipped part of the test suite:

	Before	After
Rubies tested	3.1, 3.4 (Buildkite)	3.1, 3.2, 3.3, 3.4, 4.0
Platforms	Linux, Windows	Linux, Windows, macOS
Cucumber features	never run	ported to RSpec, run everywhere

New unit workflow

.github/workflows/unit.yml runs rake spec on a matrix of Ruby 3.1-4.0 × ubuntu-latest, windows-latest and macos-latest, using ruby/setup-ruby with bundler caching. Chef's Buildkite setup has no Windows image for Ruby 3.2 or 4.0 (rubydistros/windows-2019 publishes only 3.1, 3.3 and 3.4), so GitHub Actions is the only way to cover them. BUNDLE_WITHOUT: debug:docs skips pry/byebug and yard, which the specs don't need.

The Buildkite pipeline is unchanged. Its check is probably required by branch protection. Once this workflow has proven itself, maintainers may want to retire it.

Cucumber → RSpec

features/ had four scenarios. None of them had run in years: cucumber isn't in the Gemfile or gemspec, no rake task or CI job called it, and one step definition file was empty. The scenarios are ported to spec/mixlib/config_usage_spec.rb, and features/ is removed. The file-loading scenario is now the only spec that reads a real config file from disk (spec/fixtures/sample_config.rb). Every other from_file spec stubs the read.

Rakefile

Both require "bundler/gem_tasks" and Bundler::GemHelper.install_tasks were present, so build and release each ran their actions twice (Rake::Task["build"].actions.size was 2). The duplicate is removed. This PR also fixes the "uploaeded" typo, because spellcheck flags any file a PR touches. #143 makes the identical fix.

Not changed: minimum Ruby

required_ruby_version stays at >= 3.1. Ruby 3.1 is EOL, but Chef Infra Client 18 ships it, and raising the floor would cut Chef 18 off from future mixlib-config releases. The matrix now tests that floor alongside every newer Ruby.

Merge notes

This PR and #144 both edit .github/copilot-instructions.md in the directory-tree section, so whichever merges second will need a small rebase. The added cspell entry (Rovo) is identical in both PRs.

Testing

• bundle exec rake spec passes locally on Ruby 4.0.7 (160 examples). cookstyle, actionlint and cspell are clean.
• To confirm the ported file-loading spec really reads the file, I changed the fixture's value and watched the spec fail.

[jaymzh]

Buildkite is formally deprecated at Progress. Theoretically they're not going to move to Drone, but I think this is clearly the "right thing" at least for now.

Go ahead and kill the BK in this PR and then @tpowell-progress can nuke the BK requirment when he merges this.