[CI] Windows Integration loses VSM-XAM-147 agent heartbeat

## Failure signature

The **Windows Integration Tests** `Dotnet tests` job is abandoned when Azure DevOps loses contact with `VSM-XAM-147`:

```text
We stopped hearing from agent VSM-XAM-147. Verify the agent machine is running and has a healthy network connection.
```

## Confirmed occurrences

### PR #26020

- Head: `52aa6b725350ec79b8019d3d5e8523c817be9b27`
- [Build 15180927, Windows Integration Tests / Dotnet tests](https://dev.azure.com/devdiv/DevDiv/_build/results?buildId=15180927&view=logs&j=eac261bd-81a1-5958-df8d-94842fc987be)
- Azure abandoned the job after losing the agent heartbeat. A full Windows-stage attempt 2 is active on a different worker.

### PR #26240

- Head: `69ff5adf1625c166175164e1c040e45d2defca35`
- [Build 15181530, Windows Integration Tests / Dotnet tests](https://dev.azure.com/devdiv/DevDiv/_build/results?buildId=15181530&view=logs&j=eac261bd-81a1-5958-df8d-94842fc987be)
- Azure reported the same `VSM-XAM-147` heartbeat-loss message and marked the job abandoned before tests produced a result.

First observed: `2026-09-02T07:59:57Z`.
Latest confirmed occurrence: `2026-09-02T11:09:57Z`.
Recurrence: **2 jobs across 2 distinct monitored PRs**.

## Classification and impact

This is a CI infrastructure/agent failure, not a source or deterministic test failure. The same worker heartbeat symptom affects unrelated PRs and prevents the Windows test job from completing.

## Investigation performed

- Inspected both Azure build timelines, stage/job states, agent identifiers, and exact error text.
- Confirmed the source changes in the affected PRs are unrelated to Azure agent connectivity.
- Searched all open and closed `dotnet/macios` issues separately using the exact error, worker name, Windows stage, and agent-network wording. No matching issue existed.

## Workaround

Rerun the entire **Windows Integration Tests** stage, never only the failed job, after the build is terminal and mergeability/retry-budget gates permit it. Prefer a healthy worker rather than retrying on `VSM-XAM-147`.

Azure diagnostic retention is limited.

Start with the Azure build timelines for builds 15180927 and 15181530 and the Windows Integration Tests / Dotnet tests job. Compare the VSM-XAM-147 heartbeat failures with the active retry on another worker and inspect the available Azure agent or network diagnostics. Done means the Windows stage completes reliably without the recurring abandoned job, or the issue has a documented infrastructure fix.

Written by the indexing model from the issue text.