During the WT-18585 review, it was spotted that the stable table content assertion:

WT_ASSERT_ALWAYS(session, txn->time_point.commit_timestamp <= step_down_ts,
"stable content committing above the step-down timestamp"); 

should be checking the durable timestamp instead of the commit timestamp. Checking the commit timestamp will fail when we have prepared transactions.

Another good idea to consider as part of this ticket is whether these checks should be moved to set_timestamp(), where we set the commit or durable timestamp, rather than keeping them in commit_transaction(). That way, we can check the timestamps immediately as they are set.