Summary

Update the test/format framework so it can trigger and support asynchronous step-downs. This is an upgrade from the synchronous (elegant) step-down support.

Motivation

test/format is the primary randomized stress/fuzz test harness for WiredTiger. Extending it to exercise async step-down ensures the core step-down path is validated under randomized concurrent workloads without requiring hard node restarts.

Scope

• Add async step-down trigger and handling in test/format
• Validate that existing test/format runs succeed with async step-down enabled

This ticket explicitly excludes:
- Python test suite coverage (separate ticket)

Definition of Done

• test/format consistently executes and validates async step-down runs without requiring hard node restarts