Fix IllegalStateException in owner pets mapping-created-by-agentic

[doppleware]

An IllegalStateException is occurring in the /owners/{ownerId}/pets endpoint when attempting to map pet IDs to owner IDs. The error indicates a duplicate key conflict when trying to merge values for owners with multiple pets.

Error details:

• Error ID: bebfa1a8-3ba0-11f0-a505-42975a299f33
• Type: IllegalStateException
• Location: Collectors.duplicateKeyException
• Endpoint: /owners/{ownerId}/pets
• Error message: Duplicate key 3 (attempted merging values [3] and [4])

Root cause:
The current implementation uses Collectors.toMap() which doesn't support multiple values for the same key, but the database structure allows multiple pets per owner.

Fix:
A PR has been created (#96) that modifies the stream collection logic to use Collectors.groupingBy instead, which properly handles the one-to-many relationship between owners and pets.

Impact:
This issue causes the /owners/{ownerId}/pets endpoint to fail when attempting to retrieve pets for owners who have multiple pets.