[WinUI OSS] Investigate validation support for GitHub-backed development

## Summary

Investigate validation options for GitHub-backed WinUI changes, including whether maintainers need a manual validation path in addition to normal PR checks.

## Context

Engineers may need to validate changes before or beyond standard PR validation. The ADO title references buddy builds, but the public issue should describe the need generically: how maintainers request deeper validation for GitHub-sourced changes.

## Work to do

- Identify validation paths available for GitHub-sourced changes.
- Determine whether PR checks are sufficient for normal development.
- Document when maintainers should use deeper validation.
- Track gaps in manual or scheduled validation support.
- Keep private trigger commands, pipeline URLs, and environment details out of public text.

## Definition of done

- Supported validation paths are understood.
- Maintainers know how to reason about deeper validation needs.
- Unsupported scenarios are tracked.
- Public issue content does not expose private execution details.