input: Write a python function to retrieve person data using the Google App Engine ndb module.
  assert get_person_by_id('12345') == '{"name": "John Doe", "age": 30}'
  assert get_person_by_id('67890') == '{"name": "Jane Smith", "age": 25}'
  assert get_person_by_id('00000') == '{"error": "Person not found"}'
output: from google.appengine.ext import ndb
  import json
  
  class Person(ndb.Model):
      name = ndb.StringProperty()
      age = ndb.IntegerProperty()
  
  def get_person_by_id(person_id):
      person_key = ndb.Key('Person', person_id)
      person = person_key.get()
      
      if person:
          return json.dumps({"name": person.name, "age": person.age})
      else:
          return json.dumps({"error": "Person not found"})