feat: #3585451 UserSearch selection handler does not search by email (only by username)

The commerce:user selection handler was intended to allow searching by both username and email, but the buildEntityQuery() method called DefaultSelection::buildEntityQuery(), which forced a mandatory condition on the name field. This made the subsequent OR condition on mail ineffective – users could only be found by username.

Now the query is built from scratch with a proper OR condition group on both name and mail, allowing email-only searches to return results as expected.

Closes #3585451