[Pipeline Bug]: "Validation-Unattended-Failed" is sporadically given to PRs whose InstallerUrl-s are all portable and not meant for installs.

Based on:
* #342043.

It's a matter of automated taggings that contradict the manifest YAML-s' contents: Errors about silent installation failures surely can't be correct when given to a manifest where all InstallerUrl-s have `NestedInstallerFiles: portable`, indicating they are not meant for installations at all (whether they be silent or not).

### Proposed implementation

Make it so that if all InstallerUrl-s in a manifest are portable, then the pipelines can not apply `Validation-Unattended-Failed` errors.

Start with the behavior described in #342043 and trace the pipeline validation and automated tagging logic for manifest YAMLs. Check how `NestedInstallerFiles: portable` and multiple `InstallerUrl` entries are evaluated; done means `Validation-Unattended-Failed` is not applied when every installer is portable.

Written by the indexing model from the issue text.