Update macOS CI to use Xcode 16.4

- Updated from Xcode_16 to Xcode_16.4 for latest stable version
- Xcode 16.4 is the most recent stable version available on GitHub Actions macOS-15 runners
- This ensures builds use the latest available toolchain and SDKs

Author: kateinoigakukun

.github/workflows/test.yml

@@ -66,7 +66,7 @@ jobs:
       matrix:
         include:
           - os: macos-15
-            xcode: Xcode_16
+            xcode: Xcode_16.4
     runs-on: ${{ matrix.os }}
     steps:
       - uses: actions/checkout@v4