I believe the CI failure here will be resolved by #15564

Not specific to this PR, but I wonder if we should use some kind of singleton so that we don't have to wire preview everywhere. E.g., a global mutable preview thing that we initialize at the start of the program...

Not specific to this PR, but I wonder if we should use some kind of singleton so that we don't have to wire preview everywhere. E.g., a global mutable preview thing that we initialize at the start of the program...

Yeah, we've talked about it. Then you lose some visibility into which code paths have preview behaviors, but... could definitely be worth it.

If we merge #15548 first, we can remove most of the non-test changes, the preview feature gets passed in the base client builder once before the cli command match. For the test usages, we can set the default preview mode in BaseClientBuilder::default().

(Just going into a feature branch, can drop a bunch of it as you said if your PR lands)