PR Summary

Avoids an unhandled process-terminating exception when PSTaskJob.ChildJobs is mutated while a ForEach-Object -Parallel -AsJob job is starting.

PR Context

Fix #26160.

PSTaskJob.Start() currently enumerates the public ChildJobs list on a ThreadPool callback. That list is externally mutable, so removing an item can invalidate the enumerator and let the exception escape the ThreadPool callback. This change keeps an internal list of PSTaskChildJob objects for scheduler/state enumeration while preserving the existing public ChildJobs list for compatibility.

PR Checklist

• PR has a meaningful title
• Summarized changes
• This PR is focused on one issue
• Make sure all .h, .cpp, .cs, .ps1 and .psm1 files have the correct copyright header
• This PR is ready to merge and is not work-in-progress
• Breaking change: No
• User-facing change: Not required
• Tests added/updated

Validation

• Start-PSBuild -Clean -PSModuleRestore -UseNuGetOrg built pwsh.exe. The Windows PowerShell 5.1 build wrapper emitted post-build ConvertFrom-Json -Depth compatibility warnings after build output was produced.
• Start-PSPester -Path test/powershell/Modules/Microsoft.PowerShell.Utility/Foreach-Object-Parallel.Tests.ps1 -UseNuGetOrg -ThrowOnFailure -Terse -SkipTestToolBuild passed: 62 passed, 0 failed.