Fix serialization of context pages

Fixes #37373

app/controllers/activitypub/contexts_controller.rb

@@ -36,9 +36,8 @@ class ActivityPub::ContextsController < ActivityPub::BaseController
 
   def context_presenter
     first_page = ActivityPub::CollectionPresenter.new(
-      id: items_context_url(@conversation, page_params),
       type: :unordered,
-      part_of: items_context_url(@conversation),
+      part_of: context_url(@conversation),
       next: next_page,
       items: @items.map { |status| status.local? ? ActivityPub::TagManager.instance.uri_for(status) : status.uri }
     )
@@ -52,7 +51,7 @@ class ActivityPub::ContextsController < ActivityPub::BaseController
     page = ActivityPub::CollectionPresenter.new(
       id: items_context_url(@conversation, page_params),
       type: :unordered,
-      part_of: items_context_url(@conversation),
+      part_of: context_url(@conversation),
       next: next_page,
       items: @items.map { |status| status.local? ? ActivityPub::TagManager.instance.uri_for(status) : status.uri }
     )

app/serializers/activitypub/context_serializer.rb

@@ -3,7 +3,9 @@
 class ActivityPub::ContextSerializer < ActivityPub::Serializer
   include RoutingHelper
 
-  attributes :id, :type, :attributed_to, :first
+  attributes :id, :type, :attributed_to
+
+  has_one :first, serializer: ActivityPub::CollectionSerializer
 
   def type
     'Collection'

spec/requests/activitypub/contexts_spec.rb

@@ -24,6 +24,11 @@ RSpec.describe 'ActivityPub Contexts' do
       expect(response.parsed_body[:type])
         .to eq 'Collection'
 
+      expect(response.parsed_body[:first])
+        .to include(
+          type: 'CollectionPage',
+          partOf: context_url(conversation)
+        )
       expect(response.parsed_body[:first][:items])
         .to be_an(Array)
         .and have_attributes(size: 2)