search: lift zoekt-disabled validation
Created by: rvantonder
Stacked on https://github.com/sourcegraph/sourcegraph/pull/25417.
The fallback to unindexed search should trigger unconditionally when args.Zoekt
, i.e., the Zoekt client, is nil
. We don't need to bury this fallback in newIndexedSubsetSearchRequest
and can lift it up a call. This also simplifies an indirect control flow to trigger the fallback if <global search active but Zoekt disabled>
.