[New Feature]: Skip re-posting missing metadata comments on Re-Run

### Description of the new feature/enhancement

Some PRs are re-run one (or multiple) times to try and get them to pass the pipelines. For example - https://github.com/microsoft/winget-pkgs/pull/414872

The validation pipeline is currently posting a comment about the missing properties from the previous version on every run. Since it's the same SHA being validated again, there should be no need to re-post the comment.

### Proposed technical implementation details (optional)

_No response_

Start by tracing the validation pipeline's handling of missing-properties comments for a pull request rerun, using PR #414872 as the example. Check how the validated SHA and existing comments are identified. Done means rerunning validation for the same SHA does not post a duplicate comment, while a new SHA still receives the missing-properties comment.

Written by the indexing model from the issue text.