[Firebase AI] Add macOS / Xcode 26 testing to spm job (#15330)

Author: andrewheard

.github/workflows/common.yml

@@ -95,6 +95,9 @@ jobs:
           - os: macos-14
             xcode: Xcode_16.2
             platform: iOS
+          - os: macos-26
+            xcode: Xcode_26.0
+            platform: iOS
     runs-on: ${{ matrix.os }}
     steps:
     - uses: actions/checkout@v4