[FEATURE] Add tests covering CRD validation and JSON serialisation

[Tom-Newton]

Community Note

• Please vote on this issue by adding a 👍 reaction to the original issue to help the community and maintainers prioritize this request
• Please do not leave "+1" or other comments that do not add relevant new information or questions, they generate extra noise for issue followers and do not help prioritize the request
• If you are interested in working on this issue or have submitted a pull request, please leave a comment

What is the outcome that you are trying to reach?

Avoid future bugs similar to what #2255 fixed.

Describe the solution you would like

• Tests that ensure omitting spec or metadata in the SparkApplication fail CRD validation.
• Tests that ensure optional fields still pass CRD validation after deserialization and re-serialisation by the validating webhook.

https://github.com/kubeflow/spark-operator/blob/0c49569afb2382738aabfe6ddbf5fddc577a6681/internal/webhook/sparkapplication_defaulter_test.go achieved this but was a kind of strange way to implement it.

Describe alternatives you have considered

No tests - pretty sure we don't want that.

Additional context

This is coming from discussion during PR review #2255 (comment) and I don't want it to get lost.