Disaggregated storage needs crash recovery coverage for schema operations that span a role switch. This ticket extends the leader schema abort test from WT-17972 with a switch-mode workload phase.

Workload:

• Run the schema workload from WT-17972 as the first phase in either leader or follower mode.
• After a configurable run time, quiesce worker threads and switch roles: leader steps down to follower, or follower steps up to leader. Resume the schema workload under the new role for a second phase.
• Parent process sends SIGKILL at a random point: during the first phase, during the role switch, or during the second phase.

Deliverables:

• Extend test/csuite/schema_disagg_leader_abort/ from WT-17972 with a switch-mode flag that drives the role-switch workload phase.
• Evergreen task entry in test/evergreen_disagg.yml exercising the switch-mode flag, including a stress variant with extended run time.