Cache disabled for REST resource.

Problem/Motivation

Caching is disabled on the REST resource because an object not implementing the CacheableDependencyInterface is passed to addCacheableDependency. In fact, the object being passed is currently an array.

------ --------------------------------------------------------------------- 
  Line   src/Plugin/rest/resource/VertexAISearchResource.php                  
 ------ --------------------------------------------------------------------- 
  150    Calling addCacheableDependency($object) when $object does not        
         implement CacheableDependencyInterface effectively disables caching  
         and should be avoided.                                               
         🪪  cacheable.dependency   

Steps to reproduce

This issue is being found by phpstan in the pipelines for any MRs.

Proposed resolution

Create a CacheableMetadata object and add the url.query_args context to it.
Pass this to the resourceResponse cacheable dependency.

    // Create a cacheable metadata object.
    $cacheMetadata = new CacheableMetadata();

    // Add the query arguments context so variations cache correctly.
    $cacheMetadata->addCacheContexts(['url.query_args']);
  
    $resourceResponse->addCacheableDependency($cacheMetadata);