Remove unused ActivityPub @context values depending on response (#10378)

Fix #8078

app/lib/activitypub/serializer.rb

@@ -0,0 +1,30 @@
+# frozen_string_literal: true
+
+class ActivityPub::Serializer < ActiveModel::Serializer
+  with_options instance_writer: false, instance_reader: true do |serializer|
+    serializer.class_attribute :_named_contexts
+    serializer.class_attribute :_context_extensions
+
+    self._named_contexts     ||= {}
+    self._context_extensions ||= {}
+  end
+
+  def self.inherited(base)
+    super
+
+    base._named_contexts     = _named_contexts.dup
+    base._context_extensions = _context_extensions.dup
+  end
+
+  def self.context(*named_contexts)
+    named_contexts.each do |context|
+      _named_contexts[context] = true
+    end
+  end
+
+  def self.context_extensions(*extension_names)
+    extension_names.each do |extension_name|
+      _context_extensions[extension_name] = true
+    end
+  end
+end