Larger Context

This ticket adds the core role-relative switch_role operation for deterministic single-node switch-mode workloads. It depends on the follower-mode foundation from WT-18867.

Motivation

test/model needs one operation that changes the current role and can be called repeatedly from either role.

Scope

Add switch_role to the workload format, parsing, serialisation, model runner, and WiredTiger runner. Track the current role dynamically. The initial contract requires no active transactions across a transition and resets workload cursors and sessions at the boundary. Stable must cover completed writes before the transition.

The transition sequence depends on the current role:

• From leader, take a checkpoint and reconfigure to follower.
• From follower, reconfigure to leader and take a checkpoint.

Add fixed workloads for each direction and repeated transitions. Add workload validation and counterexample-reduction handling for switch_role.

Acceptance Criteria

• Fixed workloads pass for each direction and repeated transitions.
• No active transaction, cursor, or workload session crosses a transition.
• Stable and checkpoint operations occur in the required order for the current role.
• Model and WiredTiger return codes and final recovered table contents match.
• Reduced workloads contain valid role-relative switch_role operations.

Outcome

Test/model provides one synchronous role-transition operation for deterministic single-node workloads.