[CI Failure] dnceng NuGet flat2 package downloads time out (no data received for 60000ms)

## Build Information

Build: https://github.com/microsoft/aspire/actions/runs/31269165475
Build error leg: Tests / Azure.Messaging.ServiceBus / Azure.Messaging.ServiceBus (ubuntu-latest)
Pull request: #19139

## Error Message

```
Failed to download package '' from 'https://pkgs.dev.azure.com/dnceng/.../_packaging/.../nuget/v3/flat2/.../*.nupkg'. An error occurred while sending the request. The download of '...' timed out because no data was received for 60000ms. The operation has timed out.
```

## Description

dnceng NuGet flat2 package downloads time out (no data received for 60000ms)

**Type**: infra-failure

## Occurrences

| Date | Build | Job | PR |
|------|-------|-----|----|
| 2026-08-08 | [31269165475](https://github.com/microsoft/aspire/actions/runs/31269165475) | Tests / Azure.Messaging.ServiceBus / Azure.Messaging.ServiceBus (ubuntu-latest) | #19139 |
| 2026-08-18 | [32093154322](https://github.com/microsoft/aspire/actions/runs/32093154322) | Tests / Templates-MSTest_NewUpAndBuildSupportProjectTemplatesTests / Templates-MSTest_NewUpAndBuildSupportProjectTemplatesTests (macos-latest) | #1424 |

Start with the linked build run 31269165475 and inspect the Tests / Azure.Messaging.ServiceBus job logs for the 60000ms flat2 download timeout. Compare it with run 32093154322 and its macOS template-test job; done requires identifying and validating the cause of the repeated package-download failures.

Written by the indexing model from the issue text.