fix(install): fail fast when uv venv genuinely fails under relaxed EAP

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.

scripts/install.ps1

@@ -1460,6 +1460,15 @@ function Install-Venv {
     # PowerShell 5.1 with EAP=Stop that stderr is a NativeCommandError unless
     # we temporarily relax EAP and trust $LASTEXITCODE for real failures.
     Invoke-NativeWithRelaxedErrorAction { & $UvCmd venv venv --python $PythonVersion }
+    # Relaxing EAP above means a *genuine* uv-venv failure (exit != 0) no longer
+    # aborts on its own. Capture $LASTEXITCODE immediately and fail fast, so the
+    # `venv` stage can't falsely report success (and Invoke-Stage can't emit
+    # ok=true) when the venv was never created.
+    $venvExitCode = $LASTEXITCODE
+    if ($venvExitCode -ne 0) {
+        Pop-Location
+        throw "Failed to create virtual environment (uv venv exited with $venvExitCode)"
+    }
 
     # Neutralize any inherited UV_PYTHON (e.g. $env:UV_PYTHON = "3.14" left in
     # the user's shell). uv honours UV_PYTHON over an existing venv for the