Voice installer fails: tries private Azure Artifacts feed causing 401 for Microsoft.AI.Foundry.Local.Core 1.2.3

Summary\n\nWhen enabling voice mode, the Copilot CLI voice runtime installer attempts to download Microsoft.AI.Foundry.Local.Core v1.2.3 from a private Azure Artifacts feed and fails with HTTP 401. The package is publicly available on nuget.org, so the installer should not require Azure DevOps access or should fall back to nuget.org when a private feed returns 401.\n\nReproduction steps\n1. On macOS with .NET 10 (example environment below)\n2. Run the Copilot CLI voice command that triggers runtime installation (e.g., `copilot voice` / enable voice)\n3. Installer fails with 401 trying to download from Azure Artifacts feed\n\nObserved behavior\nVoice runtime install failed: Error: Failed to download Microsoft.AI.Foundry.Local.Core 1.2.3 from any configured feed (api.nuget.org, pkgs.dev.azure.com): Download failed with status 401: https://pkgs.dev.azure.com/aiinfra/2692857e-05ef-43b4-ba9c-ccf1c22c437c/_packaging/7982ae20-ed19-4a35-a362-a96ac99897b7/nuget/v3/flat2/microst.ai.foundry.local.core/1.2.3/microsoft.ai.foundry.local.core.1.2.3.nupkg\n\nInvestigation\n- The package is present on nuget.org: https://api.nuget.org/v3-flatcontainer/microsoft.ai.foundry.local.core/1.2.3/microsoft.ai.foundry.local.core.1.2.3.nupkg\n- The installer appears to be attempting the private pkgs.dev.azure.com feed and failing with 401, and not successfully downloading from nuget.org as a fallback.\n\nEnvironment\n- macOS (user reported)\n- dotnet --version output (example):\n10.0.301\n\nExpected behavior\n- Installer should download Microsoft.AI.Foundry.Local.Core from a public source when available (nuget.org), or gracefully fall back to public feeds when a private feed returns authentication errors.\n- The voice feature should not require Azure DevOps access for users without that service.\n\nSuggested fix\n- Prefer nuget.org or allow configuring which feed to use for voice runtime packages.\n- If a private feed is configured but returns 401, attempt nuget.org before failing the install.\n\nPlease investigate and fix.,-H:Accept: application/vnd.github+json}

Start with the voice runtime installer entry point triggered by `copilot voice` or enabling voice, and reproduce the package download on macOS with the reported .NET version. Trace how configured feeds are tried for Microsoft.AI.Foundry.Local.Core 1.2.3; done means installation succeeds from nuget.org without Azure DevOps access or handles a private-feed 401 by trying the public feed.

Written by the indexing model from the issue text.