[CI Failure] Flaky: DashboardUrlTests.GetDashboardUrlAsyncAuthenticatesDashboardBrowser expects HTTP Found but gets RedirectKeepVerb

## Build Information

Build: https://github.com/microsoft/aspire/actions/runs/34379133718
Build error leg or test failing: Tests / No-package tests (regular, Aspire.Hosting.Testing.Tests, Hosting.Testing, Hosting.Testing, tests/... / Hosting.Testing (ubuntu-latest) / `Aspire.Hosting.Testing.Tests.DashboardUrlTests.GetDashboardUrlAsyncAuthenticatesDashboardBrowser`
Pull request: #19893

## Error Message

```
Assert.Equal() Failure: Values differ
Expected: Found
Actual: RedirectKeepVerb
at Aspire.Hosting.Testing.Tests.DashboardUrlTests.GetDashboardUrlAsyncAuthenticatesDashboardBrowser() in DashboardUrlTests.cs:line 49/58
```

## Description

Flaky: DashboardUrlTests.GetDashboardUrlAsyncAuthenticatesDashboardBrowser expects HTTP Found but gets RedirectKeepVerb

**Type**: flaky-test

## Occurrences

| Date | Build | Job | PR |
|------|-------|-----|----|
| 2026-09-09 | [34379133718](https://github.com/microsoft/aspire/actions/runs/34379133718) | Tests / No-package tests (regular, Aspire.Hosting.Testing.Tests, Hosting.Testing, Hosting.Testing, tests/... / Hosting.Testing (ubuntu-latest) | #19893 |

Start in DashboardUrlTests.cs at GetDashboardUrlAsyncAuthenticatesDashboardBrowser and reproduce the failure described at lines 49/58. Compare the expected Found response with the actual RedirectKeepVerb response, then rerun this test to confirm the flaky behavior is resolved and the expected result is stable.

Written by the indexing model from the issue text.