fix: correct 404 not found responses and few test changes

[thisdudkin]

Improved handling of 404 errors for non-existing owners and pets in the POST /owners/{ownerId}/pets endpoint. Also switched to using records for error information to simplify and modernize the code.

This PR addresses #130 and enhances both the error handling and the structure of the error responses.

Changes:

• Fixed the endpoint to return a 404 Not Found status for non-existing owners or pets instead of a 400 Bad Request.
• Refactored error handling by replacing the previous class-based error representation with Java record, which stands for a more concise and immutable way to handle error details.
• Few test changes, because the old version was wrong

Please review and merge this update.