Fix Time serialization regression introduced by dropping MultiJson

JSON.dump bypasses ActiveSupport's Hash#to_json override, causing
Time values to serialize via Time#to_s ("2026-04-16 10:55:10 UTC")
instead of ActiveSupport's as_json ("2026-04-16T10:55:10.597Z").

Replace Json.dump with Hash#to_json which properly delegates through
ActiveSupport and remove the now-unused Grape::Entity::Json constant.

Fixes #403

Author: numbata

lib/grape_entity/entity.rb

@@ -1,7 +1,5 @@
 # frozen_string_literal: true
 
-require 'grape_entity/json'
-
 module Grape
   # An Entity is a lightweight structure that allows you to easily
   # represent data from your application in a consistent and abstracted
@@ -594,7 +592,7 @@ def is_defined_in_entity?(attribute)
 
     def to_json(options = {})
       options = options.to_h if options&.respond_to?(:to_h)
-      Grape::Entity::Json.dump(serializable_hash(options))
+      serializable_hash(options).to_json
     end
 
     def to_xml(options = {})

lib/grape_entity/json.rb

@@ -1880,6 +1880,23 @@ class NoPathCharacterEntity < Grape::Entity
       it 'returns a json' do
         expect(fresh_class.new(model).to_json).to eq(JSON.generate(attributes.slice(:name)))
       end
+
+      it 'serializes Time values via as_json' do
+        fresh_class.expose :birthday
+        entity = fresh_class.new(model)
+        json = entity.to_json
+        parsed = JSON.parse(json)
+        expect(parsed['birthday']).to eq(attributes[:birthday].as_json)
+      end
+
+      it 'serializes Time values in nested exposures via as_json' do
+        fresh_class.expose :details do
+          fresh_class.expose :birthday
+        end
+        entity = fresh_class.new(model)
+        parsed = JSON.parse(entity.to_json)
+        expect(parsed['details']['birthday']).to eq(attributes[:birthday].as_json)
+      end
     end
 
     describe '#inspect' do