fix: enable filtering on UserProperty fields

[yihaoWang]

Description

This PR fixes an issue where filtering queries on UserProperty fields was not working. The root cause was that the _MEANING_PREDEFINED_ENTITY_USER meaning was not being properly set at the entity's top level during query filter creation.

In the Google Datastore, UserProperty meanings are stored at the entity's top level rather than within individual property values. This PR adds the necessary handling to ensure the meaning is correctly propagated when creating filters for UserProperty fields.

Changes

• Added _comparison method to UserProperty class to set the proper meaning during filter creation
• Added documentation to clarify the special handling of UserProperty meanings at the entity level

Testing

To reproduce and verify the fix:

import os
import sys
from google.appengine.api import users
from google.cloud import ndb

class TestModel(ndb.Model):
    owner = ndb.UserProperty()


def init_ndb():
    os.environ["AUTH_DOMAIN"] = "ikala.ai"
    
    ndb_client = ndb.Client(project="cloud-sa-sandbox-1")
    return ndb_client


def test_get_user_data():
    ndb_client = init_ndb()
    with ndb_client.context():
        user = users.User(email="test@test.com")
        test_model = TestModel(owner=user)
        test_model.put()

        # Before the fix: This query would not work properly
        # After the fix: This query correctly filters by user
        query = TestModel.query().filter(TestModel.owner == user)
        result = query.get()


if __name__ == "__main__":
    test_get_user_data()

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