Create GDK runner with gitlab--duo tag

What does this merge request do and why?

This MR updates the rake runner:setup task to create a GDK runner with the gitlab--duo tag, which is now required for Remote Flows in Duo Agent Platform (added in gitlab!213431 (merged)). The task checks if a runner with this tag already exists before creating a new one to avoid duplicates.

This also fixes a bug where gdk reconfigure would run unnecessarily if no new runner was created.

Closes #3143

How to set up and validate locally

1. Run gdk rake runner:setup
2. You should see a GDK runner created with the gitlab--duo tag at https://gdk.test:3443/admin/runners

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 MR references an issue describing the change.
• This change is backward compatible. If not, include steps to communicate to users.
• Tests added for new functionality. If not, raise an issue to follow-up.
• Observability added/updated (logging, metrics, tracing).
• Documentation added/updated.
• Announcement added for notable changes.
• gdk doctor test added.