ci: run the Windows unit and Pester suites on every pull request

[tas50]

What this does

CI for this driver had two holes that mattered more here than they would elsewhere, because this plugin only ever runs on Windows.

1. The shared lint-unit workflow runs RSpec on Ubuntu only. Nothing verified the driver on the one platform it actually ships to.
2. spec/powershell holds 45 Pester tests covering the functions in support/hyperv.ps1, complete with a stub Hyper-V module so they run anywhere. They have never run in CI — only on a developer's laptop via bundle exec rake pester.

This adds two jobs:

• windows-unit — bundle exec rake test on windows-latest, Ruby 3.1 and 3.4. File.join, Dir.exist? and the WOW64 architecture detection in powershell.rb all behave differently on Windows, and none of that was exercised.
• pester — runs spec/powershell/run_tests.ps1 on windows-latest. It needs no bundle, just Pester 5, and uploads the NUnit results as an artifact.

Why there is no kitchen test job

I looked at adding real integration suites the way kitchen-docker has them, and it is not possible on GitHub-hosted runners: Hyper-V requires nested virtualization, which hosted runners do not provide, and the Hyper-V role cannot be enabled on them. Inventing a suite that cannot run would be worse than not having one, so instead the workflow now carries a comment saying exactly that, and points at the "Manual testing against Hyper-V" section of CONTRIBUTING.md, which is where end-to-end verification actually lives.

If we ever get a self-hosted Hyper-V runner, that is the point to add a kitchen test job.

Dead code removed

support/ci/windows_ci.bat and support/ci/windows_integration.bat are both gone. Nothing in the repo referenced either file. windows_ci.bat duplicated what the new jobs do, and windows_integration.bat could not have worked at all — it ran bundle exec kitchen verify windows against a kitchen.yml this repo does not have, using %machine_user% and %machine_pass% that nothing sets.

Verification

$ bundle exec rake test
120 examples, 0 failures

$ bundle exec cookstyle --chefstyle
12 files inspected, no offenses detected

$ bundle exec rake pester
Tests Passed: 45, Failed: 0, Skipped: 0