[Flaky Test] Microsoft.Build.Engine.UnitTests.MSBuildServer_Tests.TransientBuildDoesNotAdoptResidentSidecars

**Flaky-test key** (automated de-duplication — do not edit):
```text
flaky-test-id: Microsoft.Build.Engine.UnitTests.MSBuildServer_Tests.TransientBuildDoesNotAdoptResidentSidecars
```

## Evidence

The test `TransientBuildDoesNotAdoptResidentSidecars` has been observed failing intermittently across multiple independent sources.

- **Distinct sources:** 3 (2 approved PRs + 1 rolling build)
- **PRs:** #14767, #14834
- **Rolling builds:** [1560446](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1560446)
- **Distinct error signatures:** 2 — non-deterministic
- **Legs:** CoreOnMac, FullOnWindows Release
- **TFMs:** net11.0
- **Assembly:** Microsoft.Build.CommandLine.UnitTests
- **Source:** src/MSBuild.UnitTests/MSBuildServer_Tests.cs
- **First seen:** 2026-08-20 | **Last seen:** 2026-08-25
- **Sample error:** `Assert.Equal() Failure: Values differ — Expected: 1, Actual: 2`
- **Sample build:** [1560398](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1560398)

## Action

This test will be quarantined via `[ActiveIssue("https://github.com/dotnet/msbuild/issues/")]` (from `Microsoft.DotNet.XUnitV3Extensions`, namespace `Xunit`) in a follow-up automated PR, once this issue has a number. The underlying nondeterminism should be fixed by the auto-fixer workflow or a human contributor.

> [!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/33186568494) · 72 AIC · ⌖ 6.72 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 in src/MSBuild.UnitTests/MSBuildServer_Tests.cs at TransientBuildDoesNotAdoptResidentSidecars, using the linked failures from PRs #14767 and #14834 and rolling build 1560446 as reproduction evidence. Investigate the intermittent Expected: 1, Actual: 2 result on the affected platforms and TFMs. Done means the test reliably passes without the nondeterministic failure, with its existing coverage preserved.

Written by the indexing model from the issue text.