Fix CaptureStderr restoration timing in testutil to resolve CI unit test failure (#26687)

* Initial plan

* chore: start CI failure investigation

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Co-authored-by: pelikhan <4175913+pelikhan@users.noreply.github.com>

* fix: restore os.Stderr immediately in CaptureStderr helper

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Co-authored-by: pelikhan <4175913+pelikhan@users.noreply.github.com>

* fix: close CaptureStderr reader after read completion

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---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: pelikhan <4175913+pelikhan@users.noreply.github.com>

Author: Copilot

pkg/testutil/tempdir.go

@@ -80,15 +80,20 @@ func CaptureStderr(t *testing.T, fn func()) string {
 
 	origStderr := os.Stderr
 	os.Stderr = w
-	t.Cleanup(func() { os.Stderr = origStderr })
+	defer func() { os.Stderr = origStderr }()
 
 	fn()
 
-	w.Close()
+	if err = w.Close(); err != nil {
+		t.Fatalf("CaptureStderr: failed to close writer pipe: %v", err)
+	}
 	var buf bytes.Buffer
 	if _, err = buf.ReadFrom(r); err != nil {
 		t.Fatalf("CaptureStderr: failed to read pipe: %v", err)
 	}
+	if err = r.Close(); err != nil {
+		t.Logf("Warning: failed to close stderr capture reader: %v", err)
+	}
 	return buf.String()
 }