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.

tests/test_install_ps1_native_stderr_eap.py

@@ -54,6 +54,31 @@ def test_uv_venv_and_dependency_installs_relax_eap() -> None:
     _assert_relaxed_call(text, r"& \$UvCmd pip install -e \$tier\.Spec")
 
 
+def test_uv_venv_failure_is_not_swallowed_after_eap_relax() -> None:
+    """Relaxing EAP must not let a genuine `uv venv` failure pass as success.
+
+    Once EAP is relaxed, a real non-zero `uv venv` exit no longer aborts on its
+    own, so install.ps1 must capture $LASTEXITCODE right after the call and fail
+    fast — otherwise the `venv` stage falsely reports success (Invoke-Stage emits
+    ok=true) when no venv was created. Regression guard for the gap caught while
+    reviewing #48372 (the explicit check originally proposed in #48463).
+    """
+    text = _install_ps1()
+    # The uv-venv invocation, then an exit-code capture, then a throw — all
+    # within a small window after the relaxed call.
+    guard = re.search(
+        r"& \$UvCmd venv venv --python \$PythonVersion[\s\S]{0,400}?"
+        r"\$LASTEXITCODE[\s\S]{0,200}?"
+        r"-ne 0[\s\S]{0,200}?throw",
+        text,
+    )
+    assert guard is not None, (
+        "install.ps1 must capture uv venv's exit code and throw on failure after "
+        "relaxing ErrorActionPreference, so a genuine venv-creation failure isn't "
+        "reported as a successful stage"
+    )
+
+
 def test_native_eap_helper_always_restores_previous_preference() -> None:
     text = _install_ps1()
     m = re.search(