[Vertex AI] Run visionOS tests on macOS 13

Author: andrewheard

.github/workflows/vertexai.yml

@@ -19,10 +19,18 @@ jobs:
     strategy:
       matrix:
         target: [iOS, macOS, catalyst, tvOS, visionOS, watchOS]
-        os: [macos-14]
+        os: [macos-13, macos-14]
         include:
+          - os: macos-13
+            xcode: Xcode_15.2
+            target: visionOS
           - os: macos-14
             xcode: Xcode_15.2
+        exclude:
+          # visionOS will be unsupported on macOS 14 as of September 23, 2024:
+          # https://github.com/actions/runner-images/issues/10559
+          - os: macos-14
+            target: visionOS
     runs-on: ${{ matrix.os }}
     env:
       FIREBASECI_USE_LATEST_GOOGLEAPPMEASUREMENT: 1