[Firebase AI] Add macOS / Xcode 26 testing to spm job

Author: andrewheard

.github/workflows/firebaseai.yml

@@ -27,6 +27,12 @@ permissions:
 
 jobs:
   spm:
+    strategy:
+      matrix:
+        include:
+          - os: macos-26
+            xcode: Xcode_26.0
+            platform: iOS
     uses: ./.github/workflows/common.yml
     with:
       target: FirebaseAIUnit