Optimize extend_safe function performance using set-based lookups

[allrob23]

Description

This PR optimizes the extend_safe function by replacing linear searches with constant-time set lookups, reducing time complexity from O(n*m) to O(n+m).

Changes

• Modified extend_safe to use a set for checking element existence
• Maintain a set copy of the target list to enable O(1) lookups
• Update the set as new elements are added to keep it in sync

Performance Impact

• Before: O(n*m) where n = len(target) and m = len(source)
• After: O(n+m) with constant-time membership checks

Testing

• Tested with various input sizes to verify correctness and performance improvement
• Maintains identical functionality while significantly improving execution time for large lists