[New Feature]: Validate installer availability in Validation Pipeline during package version removal

### Description of the new feature/enhancement

When a PR is submitted to remove a package version, the Validation Pipeline can check if the installer(s) are still available. If an installer is still available, a warning can be posted to the pull request.
This warning should not block the PR from merging, as there are many valid reasons for removal even if the installer is still available (e.g., move this package version to another package identifier.).
I'm aware that Stephen's automation already performs a similar check (e.g., https://github.com/microsoft/winget-pkgs/pull/289633#issuecomment-3250874699), but I think if incorporating it into the validation pipeline might provide faster, immediate feedback to the author.

### Proposed technical implementation details (optional)

_No response_

Start by locating the repository's Validation Pipeline and review the existing installer-availability automation referenced in the issue. Define how package-version removal is detected, where a non-blocking pull request warning should appear, and how the behavior can be tested; the work is done when valid removals remain mergeable while available installers produce immediate feedback.

Written by the indexing model from the issue text.