[#30405] Adding tests for QueryExplanation - Backwards compatability of serialization

[mirkojotic]

Solves #30405

I might have gone in a completely wrong direction here and I'm willing to work on it. But in essence the way I understood the problem is there were no tests asserting that versions older than 6.4.0 would be using a different StreamInput / StreamOutput methods than on or after it.

[elasticmachine]

Pinging @elastic/es-search-aggs

[javanna]

@pcsanwald would you like to have a look at this one given that it solves an issue that you originally opened?

[pcsanwald]

@javanna this addresses the original problem that @jpountz pointed out in #30390, which is that nothing broke when we changed the backward compat requirements for QueryExplanation. I tested this PR by pulling the branch, ensuring test passes, changing the compat string in QueryExplanation to Version.V_6_5_0, and ensuring that the test then fails:

> Task :server:test FAILED
Tests with failures:
  - org.elasticsearch.action.admin.indices.validate.query.QueryExplanationTests.testCompatabilityOfStreamInputApi_V_6_4_0
  - org.elasticsearch.action.admin.indices.validate.query.QueryExplanationTests.testCompatabilityOfStreamOutputApi_V_6_4_0

[pcsanwald]

Tested compatibility changes and it behaves as we'd expect. Thank you for contributing this!

[polyfractal]

@elasticmachine test this

[cbuescher]

@pcsanwald going through open PRs, this one seems outdated at least when targeting master.
The serialization for different versions tested here is no longer there on master. We could backport
the V_6_4_0 tests to 7.x but I'm not sure this is necessary. Would you be okay to close?

@mirkojotic sorry this slipped through the cracks somehow. Might backport this partially to 7.x if @pcsanwald
thinks that makes sense.

[pcsanwald]

ok to close, thanks @cbuescher