CI Tests: PowerShellEditorServices build cannot restore .NET reference packs (NuGet feed returns 401), failing every branch since 2026-06-25

CI Tests has failed on every branch since 2026-06-25, including main (the last successful run was 2026-06-25). It fails in the Build, test and package step while building the sibling PowerShellEditorServices checkout, before any of this repository's own tests run:

error : Failed to download package 'Microsoft.NETCore.App.Ref.8.0.31' from 'https://pkgs.dev.azure.com/powershell/.../nuget/v3/flat2/microsoft.netcore.app.ref/8.0.31/microsoft.netcore.app.ref.8.0.31.nupkg'.
error : Response status code does not indicate success: 401 (Unauthorized - No local versions of package 'microsoft.netcore.app.ref'; please provide authentication to access versions from upstream that have not yet been saved to your feed.)

PowerShellEditorServices/nuget.config clears all sources and only lists https://pkgs.dev.azure.com/powershell/PowerShell/_packaging/powershell/nuget/v3/index.json. That feed's index.json is public (HTTP 200), but package downloads return 401 to anonymous clients, for both the name-based and GUID-based URLs, e.g. microsoft.netcore.app.ref 8.0.19 and 8.0.31. The same package is publicly downloadable from nuget.org (HTTP 302). Since ci-test.yml checks out PSES's default branch and builds it, public GitHub runners can no longer restore these .NET reference packs.

Effect: no PR can get a green CI Tests run; several PRs (e.g. #5558) are stuck on this.

Workaround that unblocks a run — add nuget.org to the sources after checking out PSES (see the temporary commit in #5558):

- name: Add nuget.org to the PowerShellEditorServices sources
  shell: pwsh
  run: |
    # keep the PowerShell feed, add nuget.org as a fallback for the ref packs

With it, the build succeeds and the extension's tests actually execute (the remaining macOS failure in that run was the unrelated, flaky VS Code Insiders download: spawn ... Electron ENOENT).

Suggested fixes (maintainer's call):

1. Add nuget.org to PowerShellEditorServices/nuget.config, or enable anonymous upstream access on the feed.
2. Check out a pinned PSES tag in ci-test.yml rather than its default branch.

Start with ci-test.yml and the checked-out PowerShellEditorServices/nuget.config; trace how the Build, test and package step restores the .NET reference packs. Apply the maintainer-approved source or checkout fix, then rerun CI Tests and confirm the extension tests execute with a successful restore.