C# Add support for authenticated nuget feeds

**Description of the issue**

This is using advanced mode with `build-mode: none` to take advantage of the "no build" csharp analyzers.

When using authenticated nuget feeds with `build-mode: none`, CodeQL will try and restore the packages, however there are multiple errors within the logs indicating that the feed failed due to 401 errors:

```
[2026-03-20 02:10:31] [build-stdout] Checking if NuGet feed 'https://my-artifactory-feed' is reachable...
[2026-03-20 02:10:31] [build-stdout] Querying NuGet feed 'https://my-artifactory-feed' failed in a timely manner. Not considering the feed for use. The reason for the failure: Response status code does not indicate success: 401.
```

Start by tracing the advanced-mode path for C# analysis with build-mode: none, focusing on where NuGet feeds are checked and packages are restored. Determine how authenticated feed credentials are expected to reach that path; done means authenticated feeds can be used without the reported 401 failures.

Rédigé par le modèle d'indexation à partir du texte de l'issue.