[WinUI OSS] Automate WinUI Gallery build and publishing validation

Summary

Track automation needed for WinUI Gallery build, validation, and publishing readiness so updates do not depend on manual release steps.

Context

WinUI Gallery should have a reliable automation path for validating changes and preparing updates. The public tracker should describe the desired outcome at a high level: repeatable builds, clear validation signal, and documented publishing readiness.

Avoid naming private pipeline definitions, queue names, publishing credentials, store-management details, or internal release infrastructure in public issue text.

Work to do

• Identify the current manual steps for validating and preparing WinUI Gallery updates.
• Define the build and validation stages that should be automated.
• Track gaps in packaging, signing, publishing readiness, and release notes as follow-up tasks where needed.
• Document how maintainers should interpret automation results.
• Keep credentials, private service links, and internal release mechanics out of the public issue.

Definition of done

• Required WinUI Gallery automation stages are understood.
• Build and publishing-readiness gaps are represented by concrete follow-up issues.
• Maintainers have a clear public-safe view of the desired automation flow.
• Public issue text does not expose private release infrastructure or credentials.