[CI Failure] dnceng NuGet feed returns HTTP 503 Service Unavailable during package restore

## Build Information

Build: https://github.com/microsoft/aspire/actions/runs/30906478483
Build error leg: Tests / Hosting-1 / Hosting-1 (windows-latest)
Pull request: #1056

## Error Message

```
Response status code does not indicate success: 503 (Service Unavailable).
Failed to download package '...' from 'https://pkgs.dev.azure.com/dnceng/9ee6d478-d288-47f7-aacc-f6e6d082ae6d/_packaging/45bacae2-5efb-47c8-91e5-8ec20c22b4f8/nuget/v3/flat2/...'. Response status code does not indicate success: 503 (Service Unavailable).
```

## Description

dnceng NuGet feed returns HTTP 503 Service Unavailable during package restore

**Type**: infra-failure

## Occurrences

| Date | Build | Job | PR |
|------|-------|-----|----|
| 2026-08-04 | [30906478483](https://github.com/microsoft/aspire/actions/runs/30906478483) | Tests / Hosting-1 / Hosting-1 (windows-latest) | #1056 |

Start with build 30906478483 and the Tests / Hosting-1 / Hosting-1 (windows-latest) leg, then inspect the package-restore logs and the dnceng feed URL shown in the error. Done means determining an actionable cause or mitigation for the HTTP 503 and confirming that the affected restore succeeds.

Written by the indexing model from the issue text.