fix(vertexai): make contents_delta_uri a required field in google_vertex_ai_index (#9066)

Author: shotarok

mmv1/products/vertexai/Index.yaml

@@ -100,6 +100,7 @@ properties:
     properties:
       - !ruby/object:Api::Type::String
         name: 'contentsDeltaUri'
+        required: true
         description: |-
           Allows inserting, updating  or deleting the contents of the Matching Engine Index.
           The string must be a valid Cloud Storage directory path. If this