Fix duplicate pet ID handling - created by agentic

[shaykeren]

This issue tracks the changes needed to fix the duplicate pet ID problem that causes IllegalStateException errors.

Problem:

• The application currently allows pets with duplicate IDs to be added to an owner
• This causes IllegalStateException when trying to retrieve pets

Solution:

1. Added validation in Owner.addPet() to prevent duplicate Pet IDs
2. Added a unique constraint on the pet_id column in the database
3. Added error handling for duplicate ID scenarios

Related PR: #57