[Flaky Test] Microsoft.Build.Tasks.UnitTests.WriteLinesToFile_Tests.TransactionalModeHandlesConcurrentWritesSuccessfully

**Flaky-test key** (automated de-duplication — do not edit):
```text
flaky-test-id: Microsoft.Build.Tasks.UnitTests.WriteLinesToFile_Tests.TransactionalModeHandlesConcurrentWritesSuccessfully
```

## Evidence

- **Distinct sources:** 3 (3 rolling main builds)
- **Affected PRs:** none
- **Rolling build IDs:** [1496519](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1496519), [1506834](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1506834), [1514373](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1514373)
- **Legs / TFMs:** FullOnWindows, FullOnWindows Release / net472
- **Assembly:** Microsoft.Build.Tasks.UnitTests
- **Fully-qualified test name:** Microsoft.Build.Tasks.UnitTests.WriteLinesToFile_Tests.TransactionalModeHandlesConcurrentWritesSuccessfully
- **First seen:** 2026-07-07 | **Last seen:** 2026-07-17
- **Distinct error hashes:** 2 (indicates nondeterministic failure)
- **Sample error:**
```
Microsoft.Build.Exceptions.BuildAbortedException : Build was canceled. Failed to successfully launch or connect to a child MSBuild.exe process.
```
- **Sample build:** https://dev.azure.com/dnceng-public/public/_build/results?buildId=1496519

## Notes

This test will be quarantined via `[ActiveIssue("")]` (from `Microsoft.DotNet.XUnitV3Extensions`, namespace `Xunit`) on a follow-up automated run once this issue has a real number. The quarantine stamps `Category=failing`, which normal CI excludes, and the quarantine pipeline (def 344) re-runs it on all platforms to gather signal over time. The underlying nondeterminism should be investigated and fixed by the auto-fixer workflow or a human.

> [!WARNING]
>
> Firewall blocked 1 domain
>
> The following domain was blocked by the firewall during workflow execution:
>
> - `awmgmcpg`
>> To allow these domains, add them to the `network.allowed` list in your workflow frontmatter:
>
> ```yaml
> network:
> allowed:
> - defaults
> - "awmgmcpg"
> ```
>
> See [Network Configuration](https://github.github.com/gh-aw/reference/network/) for more information.
>
>

> Generated by [Flaky Test Triage](https://github.com/dotnet/msbuild/actions/runs/29579390833) · 108 AIC · ⌖ 7.61 AIC · ⊞ 13.5K · [◷](https://github.com/search?q=repo%3Adotnet%2Fmsbuild+is%3Aissue+%22gh-aw-workflow-call-id%3A+dotnet%2Fmsbuild%2Fflaky-test-detector.agent%22&type=issues)

Start by locating Microsoft.Build.Tasks.UnitTests.WriteLinesToFile_Tests.TransactionalModeHandlesConcurrentWritesSuccessfully and reviewing the failing FullOnWindows net472 build results 1496519, 1506834, and 1514373. Reproduce the concurrent transactional-write failure and determine why child MSBuild.exe launch or connection is nondeterministic. Done means the test passes reliably in repeated runs without quarantine.

Written by the indexing model from the issue text.