Function update_data_token_count 

pub async fn update_data_token_count(
    db: &DatabaseConnection,
    data_id: Uuid,
    token_count: i64,
) -> Result<(), DatabaseError>

Update only the token_count column for a Data record.

Mirrors the Python update_document_token_count() in cognee/tasks/documents/extract_chunks_from_documents.py.