Add a diagnostic to ensure Ruby and XCode 16 are configured properly

What does this merge request do and why?

We've seen a number of issues with XCode 16 Command Line Tools. This commit adds a diagnostic to help troubleshoot these issues.

Relates to #2222 (closed)

How to set up and validate locally

1. Check out this branch.
2. On macOS, you could run mkdir -p /Library/Developer/CommandLineTools/usr/include/c++/v1 (but be sure to delete it later!).
3. Run gdk doctor.

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.