PR #48372 relaxes EAP=Stop around the uv venv call so PowerShell 5.1
doesn't mistake uv's 'Using CPython ...' stderr for a terminating
NativeCommandError. But relaxing EAP also means a *genuine* uv venv
failure (exit != 0) no longer aborts on its own — Install-Venv would
continue and print 'Virtual environment ready', and in stage mode
Invoke-Stage would report ok=true, even though no venv was created.
Capture $LASTEXITCODE immediately after the relaxed call and throw on
non-zero (Pop-Location first, matching the function's other exit paths),
so the venv stage fails fast instead of falsely succeeding. This is the
explicit guard originally proposed in #48463 (devorun), composed on top
of #48372's reusable helper + regression test.
Adds a regression test asserting the uv venv exit-code capture + throw.
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