[FIX] return unmodifiable collection from TopicPartitionVO#getReplicas

[AKakkar28]

This PR fixes an encapsulation issue in TopicPartitionVO#getReplicas() by returning an unmodifiable collection instead of exposing the internal LinkedHashMap.values().

Changes:
Wrapped replicas.values() with Collections.unmodifiableCollection.
Added TopicPartitionVOTest to verify that external mutation attempts throw UnsupportedOperationException.

[AKakkar28]

@Bert-R I have created this PR.
Its a very small change and I have a test to verify it as well.
Can you please review and merge?
Thanks

[AKakkar28]

Hi @Bert-R @davideicardi
Can someone plz review this PR and merge it thanks?

[Bert-R]

We will, when we get to it

[AKakkar28]

@davideicardi I have added another commit it is fairly simple. I added null-safety to constructors and setters and also unit tests for TopicVO and ConsumerPartitionVO.
Can you please review this thanks a lot?

[Bert-R]

@AKakkar28 Please revert the last commit. There are better ways to improve the maintainability of these value objects.

[AKakkar28]

@Bert-R Done. Should I add JUnit test for other model classes to ensure correct behavior?

[Bert-R]

I just merged this PR. Thanks for the contribution!

@Bert-R Done. Should I add JUnit test for other model classes to ensure correct behavior?
I don't think that adds a lot of value right now.