Fix local collection uri not being serialized in REST API responses (#38645)

app/serializers/rest/collection_serializer.rb

@@ -13,6 +13,10 @@ class REST::CollectionSerializer < ActiveModel::Serializer
     object.id.to_s
   end
 
+  def uri
+    ActivityPub::TagManager.instance.uri_for(object)
+  end
+
   def description
     return object.description if object.local?
     return if object.description_html.nil?

spec/serializers/rest/collection_serializer_spec.rb

@@ -29,6 +29,7 @@ RSpec.describe REST::CollectionSerializer do
     expect(subject)
       .to include(
         'account_id' => collection.account_id.to_s,
+        'uri' => include('2342'),
         'id' => '2342',
         'name' => 'Exquisite follows',
         'description' => 'Always worth a follow',