ES: cover snippet description in title search

Reindexing required

changelogs/unreleased/199220-snippet-search.yml

 ---
-title: Include snippet description as part of snippet title search (when Elasticsearch is not enabled)
+title: Include snippet description as part of snippet title search (basic search).
 merge_request: 25961
 author:
 type: added

ee/changelogs/unreleased/207355-es-add-snippet-desc.yml

+---
+title: Include snippet description as part of snippet title search (advanced search, Elasticsearch reindexing required).
+merge_request: 26870
+author:
+type: added

ee/lib/elastic/latest/snippet_class_proxy.rb

@@ -4,7 +4,7 @@ module Elastic
   module Latest
     class SnippetClassProxy < ApplicationClassProxy
       def elastic_search(query, options: {})
-        query_hash = basic_query_hash(%w(title file_name), query)
+        query_hash = basic_query_hash(%w(title description file_name), query)
         query_hash = filter(query_hash, options)
 
         search(query_hash, options)

ee/lib/elastic/latest/snippet_instance_proxy.rb

@@ -15,6 +15,7 @@ module Elastic
           :title,
           :file_name,
           :content,
+          :description,
           :created_at,
           :updated_at,
           :project_id,

ee/spec/lib/elastic/latest/snippet_instance_proxy_spec.rb

@@ -13,6 +13,7 @@ describe Elastic::Latest::SnippetInstanceProxy do
         id: snippet.id,
         title: snippet.title,
         file_name: snippet.file_name,
+        description: snippet.description,
         content: snippet.content,
         created_at: snippet.created_at,
         updated_at: snippet.updated_at,

ee/spec/models/concerns/elastic/snippet_spec.rb

@@ -131,6 +131,7 @@ describe Snippet, :elastic do
       'file_name',
       'content',
       'created_at',
+      'description',
       'updated_at',
       'state',
       'project_id',