Fix duplicate key exception in owner pets mapping-created-by-agentic

[doppleware]

This PR fixes an IllegalStateException that occurs when trying to map pets to owners when an owner has multiple pets.

Problem:
The current implementation uses Collectors.toMap() which fails with a duplicate key exception when an owner has multiple pets, as it tries to create a map with owner ID as key and a single pet ID as value.

Solution:
Changed the implementation to use Collectors.groupingBy() with Collectors.mapping() to properly handle multiple pets per owner by collecting them into lists.

Changes:

• Modified the stream collector in getOwnerPetsMap() to properly group multiple pets per owner
• Replaced Collectors.toMap() with Collectors.groupingBy() and Collectors.mapping()

Testing:
This change should be tested with:

1. Owners with single pets
2. Owners with multiple pets
3. Owners with no pets

Fixes the IllegalStateException: "Duplicate key 3 (attempted merging values [3] and [4])"