Extends WT-17975 with crash recovery coverage for the leader kill scenario, where the follower detects the leader's death and steps up.

Add KILL_LEADER mode to the test from WT-17975:

• Parent sends SIGKILL to the leader.
• Follower detects pipe EOF, steps up: reconfigures to leader role, sets stable timestamp, takes a checkpoint, and writes a sentinel file.
• Parent waits for the sentinel, lets the stepped-up follower run briefly as the new leader, then sends SIGKILL.
• Verify the stepped-up follower home using the same epoch-cutoff logic from WT-17972.

Deliverables:

• Extend test/csuite/schema_disagg_leader_abort/ from WT-17975 with KILL_LEADER kill mode and step-up logic in workload.c and main.c.
• Evergreen task entry in test/evergreen_disagg.yml covering the KILL_LEADER mode, including a stress variant.