[New Feature]: Manifest Validation Responses

### Description of the new feature/enhancement

When a manifest is invalid (YAML syntax, WinGet Manifest Syntax, etc.) a comment on the PR should provide a more helpful message so the author can easily fix it.

We should consider printing the specific error we see during manifest validation when there is a failure, rather than just saying "Invalid manifest." This will make it easier to identify and resolve it quickly.

### Proposed technical implementation details (optional)

_No response_

No files, tests, or entry points are named. Start by locating the pull-request manifest validation logic that emits "Invalid manifest" and inspect how YAML and WinGet manifest errors are currently handled. Done means validation comments expose the specific failure details needed to correct an invalid manifest.

Written by the indexing model from the issue text.