Return GraphQL response as GdsApi::Response

This is instead of `Hash`.

In Government Frontend, we need some other information from the request
response (e.g. the cache control headers) which are not included when
the response is converted to a hash.

This is not a breaking change as the new object type accepts the same
methods as the hash.

lib/gds_api/publishing_api.rb

@@ -562,9 +562,9 @@ def get_schema(schema_name)
   #
   # @param query [String]
   #
-  # @return [Hash] A response with the result of the GraphQL query.
+  # @return [GdsApi::Response] A response with the result of the GraphQL query.
   def graphql_query(query)
-    post_json("#{endpoint}/graphql", query:).to_hash
+    post_json("#{endpoint}/graphql", query:)
   end
 
 private

test/test_helpers/publishing_api_test.rb

@@ -632,7 +632,7 @@
 
       assert_equal(
         stubbed_response.to_json,
-        api_response.to_json,
+        api_response.raw_response_body,
       )
     end
   end