refactor(integration): use execute() for INSERTs and document SET interpolation

Switch INSERT and SET statements from query() to execute() since they
don't return meaningful rows. Add comment explaining why SET uses string
interpolation (parameterized values not supported).

Author: tobyhede

packages/cipherstash-proxy-integration/src/multitenant/contention.rs

@@ -40,8 +40,9 @@ mod tests {
     /// Returns the ready-to-use client (connection setup is excluded from timing).
     async fn connect_as_tenant(keyset_id: &str) -> tokio_postgres::Client {
         let client = connect_with_tls(PROXY).await;
+        // SET doesn't support parameterized values; keyset_id is from trusted env vars
         let sql = format!("SET CIPHERSTASH.KEYSET_ID = '{keyset_id}'");
-        client.query(&sql, &[]).await.unwrap();
+        client.execute(&sql, &[]).await.unwrap();
         client
     }
 
@@ -56,7 +57,7 @@ mod tests {
             let id = random_id();
             let val = random_string();
             client
-                .query(
+                .execute(
                     "INSERT INTO encrypted (id, encrypted_text) VALUES ($1, $2)",
                     &[&id, &val],
                 )
@@ -256,7 +257,7 @@ mod tests {
             let id = random_id();
             let val = random_string();
             client_a
-                .query(
+                .execute(
                     "INSERT INTO encrypted (id, encrypted_text) VALUES ($1, $2)",
                     &[&id, &val],
                 )