Honor the globally configured platform in example groups

[tas50]

Fixes chefspec#953

Problem

The platform can be set globally instead of per example group, which is how a lot of existing cookbooks configure it from spec_helper.rb:

RSpec.configure do |config|
  config.platform = 'ubuntu'
  config.version = '20.04'
end

SoloRunner#with_default_options already falls back to that setting, but the example group helpers did not:

let(:chefspec_platform) { nil }
let(:chefspec_platform_version) { nil }

Both the preload and the Chef.node stub are gated on chefspec_platform:

chef_runner_instance.preload! if chefspec_platform
...
allow(::Chef).to receive(:node).and_return(chef_node) if chefspec_platform

So with only the global setting, custom resources are never registered and referencing one from a recipe block raises:

NoMethodError:
  undefined method 'mycookbook_greet' for an instance of Chef::Recipe

Built in resources keep working, which is what makes this hard to diagnose. Adding platform 'ubuntu' to the describe block fixes it, and several people in the issue report losing hours to it.

Fix

Default the platform and version lets to the global configuration, so the example group helpers agree with the runner.

Testing

Adds a spec_global_platform acceptance example: a custom resource, a global RSpec.configure platform, and no per example group platform.

• With the fix: 2 examples, 0 failures
• With the fix reverted: 2 examples, 2 failures (NoMethodError)
• Unit suite: 197 examples, 0 failures
• All acceptance examples pass, including spec_platform, which covers the no-platform warning path

This does not address the related wrinkle in the issue thread where a global version combines with a per example group platform (ubuntu/7.8). That is a separate bug.

[tas50]

CI on this PR is red for an unrelated reason: main's bundle cannot be installed on Linux, which breaks every job before any test runs. That is fixed separately in #56, which is green across Ruby 3.1 to 3.4. This PR should go green once #56 lands and this branch is rebased.

Locally, against a working bundle, this branch passes rake unit and the acceptance examples.