fix(ci): Handle gunzip trailing garbage warning in VM build

OpenWrt firmware images contain trailing data that gunzip reports
as "trailing garbage" with exit code 2. This is normal and the
extracted image is valid. The fix ignores the warning while still
checking that extraction produced output.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

.github/workflows/build-vm-appliance.yml

@@ -289,8 +289,16 @@ jobs:
 
           echo "📦 Source image: $IMG_FILE"
 
-          # Extract
-          gunzip -c "$IMG_FILE" > /tmp/openwrt.img
+          # Extract (ignore "trailing garbage" warning - normal for firmware images)
+          # gunzip returns exit code 2 for warnings, which we can safely ignore
+          gunzip -c "$IMG_FILE" > /tmp/openwrt.img 2>/dev/null || {
+            # Check if extraction actually produced output
+            if [[ ! -s /tmp/openwrt.img ]]; then
+              echo "❌ Failed to extract image"
+              exit 1
+            fi
+            echo "   (gunzip warning ignored - normal for firmware images)"
+          }
           IMG_SIZE=$(stat -c%s /tmp/openwrt.img)
           echo "   Size: $(numfmt --to=iec $IMG_SIZE)"