Implement basic pagination for a list

You are given:

• A zero-indexed array of items items (you can assume it fits in memory).
• An integer pageSize > 0 .

Design and implement a pagination helper that supports the following operations:

1. Initialize the paginator with the array and pageSize .
2. Given a page number pageIndex (0-based), return the subarray of items that belongs to that page.
   • Page 0 contains items with indices [0, pageSize - 1] (if they exist),
   • Page 1 contains [pageSize, 2 * pageSize - 1] , and so on.
3. If pageIndex is out of range (no items on that page), return an empty list.

Assume:

• 0 <= pageIndex
• 1 <= pageSize <= 10^5
• 0 <= len(items) <= 10^6

Define the interface and implement the core method(s) for pagination. Focus on clean indexing and handling boundary cases correctly.