C#: Only use reachable feeds when private registries are configured

[mbg]

Some customers only allow access to their own private registries and do not allow access to e.g. the public NuGet feed. This PR refactors and modifies the build-mode: none logic that tests whether feeds are reachable to also check the inherited feeds, and to only pass explicit and inherited feeds which are reachable to dotnet restore.

[Copilot]

Pull request overview

This PR refactors the NuGet feed reachability checking logic in C# dependency fetching to support scenarios where customers only have access to private registries and not to public feeds like NuGet.org. The core change is to check both explicit and inherited feeds for reachability, and only pass reachable feeds to dotnet restore.

Changes:

• Refactored feed reachability checking to test both explicit feeds (from nuget.config files in the working directory and dependabot proxy) and inherited feeds (from nuget.config files outside the working directory)
• Modified RestoreProjects to receive only reachable feeds instead of all configured feeds
• Extracted common feed reachability logic into a new GetReachableNuGetFeeds method to reduce code duplication