Function process_rows 

pub async fn process_rows<H: OutboxHandler>(
    handler: &H,
    rows: &[OutboxRow],
    base_backoff: Duration,
    max_attempts: i32,
) -> Vec<OutboxAction>

Run the handler over claimed rows and decide per-row follow-up.

Pure decision logic extracted from OutboxDrainer::drain_once so delivery semantics are unit-testable without a database.