Comparison pipeline silently skips artifacts if multiple MSFT builds are found

[ellahathaway]

In the dotnet-unified-build-compare pipeline, when gathering drops, if multiple builds exist for the same repo-level commit, no asset subdirectory is created for the corresponding repo. The comparison pipeline expects each repo to have an asset subdirectory, and it checks for a merged manifest in each one. If the subdirectory is missing entirely (rather than just the manifest), the tool does not fail—it simply proceeds.

The current example of this is the sourcelink. There are multiple builds for the same commit, which results the repo being skipped when gathering the drops.

To address this issue, the following changes are needed:

1. Gather Drops should select the most recent successful build when multiple build IDs exist for the same commit.
2. The comparison pipeline should validate that each repo listed in the source-manifest.json has a corresponding directory in the base assets directory.