Pinging @elastic/es-search (Team:Search)

Will do! Thanks for reviewing.

Pinging @javanna @jimczi so you know we reverted this. I wasn't able to figure out a good self-contained fix.

Thanks for taking care of this @jtibshirani . I did some digging and I understand better what happened. In our main branch any remote cluster expression will be intercepted and converted directly when resolving remote cluster names, hence such expressions won't ever make it to the index name expression resolver which resolves local indices names.

In our 7.x branches though, we still account for indices created in 6.x that may contain a :, hence if a remote cluster expression does not resolve to any remote cluster it ends up being treated as a local expression. What I have missed in my original PR is a test without remote clusters configured or with a remote cluster expressions that would not match any configured remote cluster. In those two scenarios the search API would also be affected by the error that was only intended for API that don't support executing against remote clusters.