[ci-scan] Build break: unable to pull Helix prerequisite image

Build Information

Build: https://dev.azure.com/dnceng-public/public/_build/results?buildId=1597901 Build error leg or test failing: runtime (Build Monitor Helix Jobs) - WasmTestOnChrome-MONO-ST-System.Runtime.Tests Pull request: https://github.com/dotnet/runtime/pull/133972

This is a post-close recurrence of #117164. That issue was closed on August 18, 2026; this build completed on September 17, 2026 and reproduced the same failure on both Linux and Windows Helix queues.

• Error Details is for readers. Paste the full exception, stack trace, or build error excerpt so the failure is understandable without opening the raw log.
• Build Analysis parses only the single JSON block under Error Message.
• ErrorMessage is a case-sensitive ordinal String.Contains substring copied verbatim from the failing log.
• Set BuildRetry to true only for a clear infrastructure retry case. ExcludeConsoleLog disables Helix console-log scanning.

Error Details

Linux retry:
Unable to pull image mcr.microsoft.com/dotnet-buildtools/prereqs:ubuntu-26.04-helix-webassembly-amd64@sha256:...
Exit Code:-4

Windows retry:
Unable to pull image mcr.microsoft.com/dotnet-buildtools/prereqs:windowsservercore-ltsc2025-helix-webassembly-amd64@sha256:...
Exit Code:-4

Both failures occurred during container provisioning before the test payload ran. The Azure DevOps test runs reported zero failed tests, and the retry work items produced no recognized test-result files.

Error Message

{
  "ErrorMessage": "Unable to pull image mcr.microsoft.com/dotnet-buildtools/prereqs",
  "ErrorPattern": "",
  "BuildRetry": true,
  "ExcludeConsoleLog": false
}

Workflow artifact: ci-failure-scan Artifact kind: kbe-verification Verified match count: 2 hits in failure.log

[!NOTE] This issue was generated with GitHub Copilot assistance.

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Summary