We currently have integration tests that cover cross-cluster search. What they don't cover is cross-cluster communication across multiple versions. We should make sure that these tests run against the previous minor version and double check their coverage (e.g. do they use the fields API? do they also cover field_caps? Which features of search and field_caps they cover?).

As a natural follow-up, it might make sense to consider reusing the existing field_caps and search tests yaml tests and run them in a CCS scenario, so that we don't need to maintain parallel CCS tests manually, as they tend to be forgotten over time.

Perhaps related to #78472 as it involves more CCS testing

Currently the steps we identified to reach better coverage are

• run the existing qa:multi-cluster-search also against the previous minor version (Expand coverage of :qa:multi-cluster-search #85281)
• extend the ability to skip yaml for certain remote cluster versions ([Tests] Enhance multi cluster search test skipping #85566) (follow up needed for Expand coverage of :qa:multi-cluster-search #85281)
• investigate if we should extend qa:multi-cluster-search to also test forward compatibility, i.e. all wire compatible local cluster versions agains a "current" remote ( Run more search Rest tests in a CCS setup #86521)
• investigate if we can re-use existing search rest tests in a multi-cluster CCS setup (Run more search Rest tests in a CCS setup #86521)
• investigate if the proxy mode in CCS is covered well in tests or if it also needs more coverage

• we decided that the current coverage is okay and added randomization of the connection mode for better coverage in (Randomize connection mode for AbstractMultiClustersTestCase #86865)