Description:

The asynchronous step-down format test variants (disagg.mode=switch disagg.stepdown_async=1, added in WT-18074) run with ops.random_cursor=0.

Random cursor operations in test/format (random.c) are not executed inside an explicit transaction, so they don't fit the snapshot-transaction expectations these tests rely on: with an asynchronous step-down, operations racing the leader transition must be attributable to a consistent snapshot for the test's validation to hold, and non-transactional random cursor reads can observe state across the step-down boundary that the test then flags as a failure.

Once random cursor reads either run inside a snapshot transaction in test/format, or the async step-down validation can tolerate non-transactional reads, remove ops.random_cursor=0 from the async task definitions in test/evergreen.yml and drop the FIXME-WT-18228 comments there.