Background

WT-18276 added the step_down_disaggregated_schema_epoch configuration to WT_CONNECTION::set_timestamp, declaring the planned step-down boundary in schema-epoch space alongside the timestamp. Publishing at or below the boundary is rejected while it is set, and the step-down asserts the stable epoch landed on it exactly.

Task

Most step-down testing still declares the boundary in timestamp space only, so the epoch-side enforcement is skipped everywhere except the targeted unit tests and the schema_disagg_abort csuite. Extend the step-down workloads that use schema epochs to supply the step-down epoch, so the boundary validation, the publish restriction and the step-down assertions run under stress:

• test/format's asynchronous step-down path, when it runs with schema epochs.
• The layered async step-down suites where a scenario uses schema epochs.
• Any future multi-node step-down workloads.

This also gives coverage ahead of the server adopting the parameter (WT-18314 makes it required once that happens).