Always stub gdk.yml in specs

What does this merge request do and why?

This stubs the gdk.yml in specs by default to avoid reading the developer’s local gdk.yml, which could pollute the test environment.

To opt out, set the :gdk_root flag on tests.

Relates to #2246 (closed)

How to set up and validate locally

1. Run bundle exec rspec
2. See no failures (unless you have openbao installed, see #2199 (closed))

Impacted categories

The following categories relate to this merge request:

• gdk-reliability - e.g. When a GDK action fails to complete.
• gdk-usability - e.g. Improvements or suggestions around how the GDK functions.
• gdk-performance - e.g. When a GDK action is slow or times out.

Merge request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise an issue to follow-up.
• Documentation added/updated, if needed.
• Announcement added, if change is notable.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.