[Flaky Test] Microsoft.Build.UnitTests.BackEnd.TaskExecutionHost_Tests.TaskExceptionHandlingTest

**Flaky-test key** (automated de-duplication — do not edit):
```text
flaky-test-id: Microsoft.Build.UnitTests.BackEnd.TaskExecutionHost_Tests.TaskExceptionHandlingTest
```

## Evidence

- **Distinct sources:** 3 (1 approved PR + 2 rolling `main` builds)
- **PRs:** #14315
- **Rolling builds:** [1510684](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1510684), [1517216](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1517216)
- **Legs/TFMs:** FullOnWindows Release / net472
- **Assembly:** Microsoft.Build.Engine.UnitTests
- **First seen:** 2026-07-15 | **Last seen:** 2026-07-20
- **Error hashes:** d1622e56, 4604fc3f, 1e0ef4b0 (3 distinct failure signatures)
- **Affected variant:** `TaskExceptionHandlingTest(exceptionType: typeof(System.OutOfMemoryException), isCritical: True)`
- **Sample build:** https://dev.azure.com/dnceng-public/public/_build/results?buildId=1510574

## Sample error

```
Shouldly.ShouldAssertException : assertion on test cleanup/teardown path
```

## Notes

The test will be quarantined via `[ActiveIssue("")]` (from `Microsoft.DotNet.XUnitV3Extensions`, namespace `Xunit`) on the next triage run, once this issue has a number. This approach stamps the `Category=failing` trait so normal CI skips the test and the quarantine pipeline (def 344) continues gathering signal.

> [!WARNING]
>
> Firewall blocked 2 domains
>
> The following domains were blocked by the firewall during workflow execution:
>
> - `awmgmcpg`
> - `southcentralus0.in.applicationinsights.azure.com`
>> To allow these domains, add them to the `network.allowed` list in your workflow frontmatter:
>
> ```yaml
> network:
> allowed:
> - defaults
> - "awmgmcpg"
> - "southcentralus0.in.applicationinsights.azure.com"
> ```
>
> 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/29745368965) · 123.6 AIC · ⌖ 6.99 AIC · ⊞ 14.8K · [◷](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 with Microsoft.Build.Engine.UnitTests and the TaskExceptionHandlingTest(exceptionType: typeof(System.OutOfMemoryException), isCritical: True) variant. Compare PR #14315 and the listed rolling builds, then reproduce on FullOnWindows Release/net472 while inspecting the cleanup or teardown failure. Done means the test's flaky failure is addressed and the relevant builds remain stable.

Written by the indexing model from the issue text.