The format-stress-test-disagg-snapshot task (added in WT-18204) is a multi-node test/format stress task living in the wiredtiger-disagg project (test/evergreen_disagg.yml). All other disagg multi-node tasks were migrated to the mainline waterfall (test/evergreen.yml) under WT-16736, and the accidental duplicates are removed under WT-18770. Move this task to test/evergreen.yml alongside the other multi-node tasks and rename it to format-stress-test-disagg-multi-snapshot-read so the name reflects the multi-node family it belongs to and the untimestamped snapshot reads it stresses.