[Inference API] Avoid potential flaky wire serialization test cases

[timgrein]

AFAIU there's some small likelihood for createTestInstance and mutateInstance to return two equal instances if you don't wrap createRandom calls with randomValueOtherThan, which would lead to a flaky test failure. I've adjusted this for all wire serialization test cases and also implemented mutateInstance where null was returned. Some of these classes were records, so we strictly don't need to test this, but we already had instances, where we've converted a record to a class and implemented equals and hashCode again manually, so preempting this.

[elasticsearchmachine]

Pinging @elastic/ml-core (Team:ML)

[davidkyle]

LGTM

Thanks @timgrein

[timgrein]

@elasticmachine update branch