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

[shaykeren]

An IllegalStateException is occurring in the GET /owners/{ownerId}/pets endpoint due to improper handling of multiple pets per owner in the stream collection operation.

Problem:
The current implementation fails when attempting to merge pet lists for owners with multiple pets (specifically observed with owner ID 6 having pets 7 and 8).

Root Cause:
The stream collection operation using toMap does not properly handle the case where an owner has multiple pets, leading to duplicate key exceptions.

Solution:
A PR has been created (#357) that implements the following fixes:

1. Replaces toMap collector with groupingBy to properly handle multiple pets per owner
2. Uses ArrayList for mutable list structures
3. Adds proper null handling
4. Fixes the stream collection logic for multiple pets

Testing:
The fix has been tested with:

• Owner ID 6 (having pets 7 and 8)
• Owners with single pets
• Owners with no pets

Please review and merge PR #357 to resolve this issue.