[CI Failure] Flaky: Dashboard DashboardDataSourceTests.RunMode_DeletesOldestRunWhenLimitIsExceeded fails with UnauthorizedAccessException on Windows during metadata file move

## Build Information

Build: https://github.com/microsoft/aspire/actions/runs/32880916910
Build error leg or test failing: Tests / Dashboard / Dashboard (windows-latest) / `Aspire.Dashboard.Tests.Model.DashboardDataSourceTests.RunMode_DeletesOldestRunWhenLimitIsExceeded`
Pull request: #19620

## Error Message

```
System.UnauthorizedAccessException : Access to the path is denied.
at System.IO.FileSystem.MoveFile(String sourceFullPath, String destFullPath, Boolean overwrite)
at Aspire.Dashboard.ServiceClient.DashboardRunStore.WriteMetadata(DashboardRunMetadata metadata, String metadataPath) in /_/src/Aspire.Dashboard/ServiceClient/DashboardRunStore.cs:line 423
at DashboardRunStore.Dispose()
```

## Description

Flaky: Dashboard DashboardDataSourceTests.RunMode_DeletesOldestRunWhenLimitIsExceeded fails with UnauthorizedAccessException on Windows during metadata file move

**Type**: flaky-test

## Occurrences

| Date | Build | Job | PR |
|------|-------|-----|----|
| 2026-08-25 | [32880916910](https://github.com/microsoft/aspire/actions/runs/32880916910) | Tests / Dashboard / Dashboard (windows-latest) | #19620 |
| 2026-09-09 | [34413952365](https://github.com/microsoft/aspire/actions/runs/34413952365) | Tests / No-package tests (regular, Aspire.Dashboard.Tests, Dashboard, Dashboard, tests/Aspire.Dashboard.T... / Dashboard (windows-latest) | #20018 |

Start with Aspire.Dashboard/ServiceClient/DashboardRunStore.cs around WriteMetadata at line 423, then run or inspect Aspire.Dashboard.Tests.Model.DashboardDataSourceTests.RunMode_DeletesOldestRunWhenLimitIsExceeded on Windows. Trace the metadata file move during Dispose and verify the test no longer produces UnauthorizedAccessException in the Windows dashboard CI job.

Written by the indexing model from the issue text.