ci: isolate hosted SDK cleanup exit code

[wiresock]

Summary

• run best-effort hosted SDK package removal through Start-Process
• inspect the cleanup process's own exit code instead of leaking it through PowerShell's $LASTEXITCODE
• preserve the warning when WinGet cannot find an uninstall registration on the disposable VM

Why

Hosted run https://github.com/wiresock/WireSockUI/actions/runs/31591854488 passed SDK installation, signature validation, WireSockUI MSI install/repair/uninstall, and all Transparent, VirtualAdapter, Kill Switch, and Amnezia lifecycle checks. The step was marked failed only after WinGet returned "No installed package found" during best-effort cleanup and left its exit code as the script's final native exit code.

Validation

• PowerShell parser validation passed
• isolated nonzero child-process probe confirmed Start-Process preserves the preceding $LASTEXITCODE
• workflow security fixtures passed
• production workflow contracts passed
• git diff --check passed

[Copilot]

Pull request overview

This PR updates the hosted SDK experiment cleanup logic to prevent best-effort WinGet uninstall failures from incorrectly propagating as the PowerShell script’s final exit code in GitHub Actions.

Changes:

• Run WinGet uninstall via Start-Process -Wait -PassThru and inspect the spawned process’s ExitCode.
• Avoid relying on (and inadvertently leaking) PowerShell’s $LASTEXITCODE during best-effort cleanup, while still warning on nonzero uninstall results.

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