Fix IllegalStateException in GET /owners/pets endpoint-created-by-agentic

[shaykeren]

An IllegalStateException is occurring in the GET /owners/pets endpoint when an owner has multiple pets. This is due to the use of Collectors.toMap() which doesn't handle duplicate keys properly.

Root Cause:

• The getOwnerPetsMap method uses Collectors.toMap() which throws IllegalStateException on duplicate keys
• Multiple pets belonging to the same owner cause duplicate owner_id keys

Fix:

• Modified the code to use Collectors.groupingBy instead of Collectors.toMap
• Added proper handling of multiple pets per owner
• Improved error handling and logging

Related PR: #90