Update file repository_xray.md

What does this MR do and why?

This update adds information about Repository X-Ray compatibility, clarifying that it works with both GitLab-hosted AI Gateway with vendor models and self-hosted GitLab Duo setups (when using supported platforms and models).

References

Screenshots or screen recordings

Before	After

How to set up and validate locally

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.