public async Task Rename(Concept concept, string newName, CancellationToken cancellationToken)
{
var existing = await _conceptRepository.GetByName(newName, cancellationToken);
if (existing != null && existing.ConceptId != concept.ConceptId)
{
throw new DomainValidationException($"A concept named {newName} already exists.");
}
concept.Rename(newName);
}