Fix #287 hide technical exception messages in API error responses (SQL / Hibernate details)

[San-43]

This PR fixes issue #287.

I reproduced the issue locally, added tests, and implemented a fix.

Changes included:

• validate owner existence before saving a pet.
• avoid exposing technical persistence details in API responses.
• add tests for the missing-owner scenario and error response sanitization.
• update testCreatePetSuccess to match the new logic.

Please, let me know if there is anything wrong.

[vfedoriv]

@San-43 can you use AssertJ equivalents instead, pls?
we already have some mix of styles/test libraries, it will be good to stay with AssertJ by default.

[San-43]

Hello @vfedoriv, thank you for your review, I'll be working on it.

[San-43]

@San-43 can you use AssertJ equivalents instead, pls? we already have some mix of styles/test libraries, it will be good to stay with AssertJ by default.

Hi @vfedoriv, I updated the test to use AssertJ equivalents, removed unnecessary imports, and adjusted the logic as well, since I believe the previous version was not quite correct. Pls let me know if there's anything else that i should change.

[San-43]

Thank you very much @vfedoriv for your review and for merging. It means a lot to me.