Check field scripts return serializable object

[rjernst]

When a script field is used, the returned value must be serializable, sometimes with transport (if there is more than one node) and always with x-content. Currently returning a non-serializable value can result in errors later. This commit more aggressively checks the value returned from field scripts to ensure a more confusing error is not encountered.

[elasticsearchmachine]

Hi @rjernst, I've created a changelog YAML for you.

[cla-checker-service]

💚 CLA has been signed

[elasticsearchmachine]

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

[rjernst]

@romseygeek I opened this PR a while ago to generally stop objects returned by doc scripts from causing serialization errors. The PR has sat stalled because other failures (ie other classes that do not have either xcontent or transport available are returned by some test scripts). I just found your recent PR #95673 which changed the behavior to have the script fallback to <unserializable>. I guess that is ok, but I think this more general purpose check is better, right next to where the script is run. Additionally, this more narrowly checks serializabilty, while other errors will continue to be propagated.

My only difficult is finding a way to test this...ScriptFieldPhase does not have unit tests, and I don't see a way to make LeafReaderContext mockable, but maybe you have ideas on how to test it?