[CI Failure] Flaky: DownloadNativeArchivesTests.FailsWhenOneArtifactDownloadFails_ButReportsAllResults fails with 'Address already in use' port conflict

## Build Information

Build: https://github.com/microsoft/aspire/actions/runs/32068234960
Build error leg or test failing: Tests / Infrastructure / Infrastructure (ubuntu-latest) / `Infrastructure.Tests.DownloadNativeArchivesTests.FailsWhenOneArtifactDownloadFails_ButReportsAllResults`
Pull request: #19440

## Error Message

```
System.Net.HttpListenerException : Address already in use
at System.Net.HttpEndPointManager.GetEPListener(String host, Int32 port, HttpListener listener, Boolean secure)
at System.Net.HttpEndPointManager.AddPrefixInternal(String p, HttpListener listener)
at System.Net.HttpEndPointManager.AddListener(HttpListener listener)
at System.Net.HttpListener.Start()
at Infrastructure.Tests.DownloadNativeArchivesTests.MockAzdoServer.Start()
```

## Description

Flaky: DownloadNativeArchivesTests.FailsWhenOneArtifactDownloadFails_ButReportsAllResults fails with 'Address already in use' port conflict

**Type**: flaky-test

## Occurrences

| Date | Build | Job | PR |
|------|-------|-----|----|
| 2026-08-17 | [32068234960](https://github.com/microsoft/aspire/actions/runs/32068234960) | Tests / Infrastructure / Infrastructure (ubuntu-latest) | #19440 |

Start with Infrastructure.Tests.DownloadNativeArchivesTests.FailsWhenOneArtifactDownloadFails_ButReportsAllResults and its MockAzdoServer.Start method, then run the test repeatedly to observe the port conflict. Trace how the mock server is started and stopped; done means the test no longer fails with “Address already in use” and still reports all download results.

Written by the indexing model from the issue text.