Function build_read_query 

fn build_read_query(safe_name: &str, limit: u32, target_cluster: &str) -> String

Builds the SQL the agent runs for read_data_product.

safe_name must already be the validated, quoted form produced by safe_data_product_name. target_cluster is escaped as a SQL string literal and wrapped in SET CLUSTER inside a BEGIN READ ONLY transaction so the cluster choice is scoped to this read and does not leak into the session.