changefeedccl: cloudstorage sink file naming may not always be lexicographically ordered

Per https://github.com/cockroachdb/cockroach/issues/155015#issuecomment-3386163179, the cloudstorage sink assumes that every time it's flushed, the changefeed's frontier timestamp has advanced. This isn't always true (e.g. when the feed is lagging and the lag checkpoint code path is hit: https://github.com/cockroachdb/cockroach/blob/94b54b74bdbc45a9aae1e74d463ed4b25e5661ae/pkg/ccl/changefeedccl/changefeed_processors.go#L959-L967) and may have ramifications for making sure that new cloudstorage sink files have names that sort lexicographically after all previously written files.

Jira issue: CRDB-55306