Fix: max_num_pages becomes negative when posts_per_page is -1

[burhandodhy]

Description of the Change

When running a WP_Query posts_per_page => -1, ElasticPress was calculating max_num_pages as ceil( found_posts / -1 ), producing a negative value (for example, -17 when 17 posts were found). WordPress core treats posts_per_page => -1 as "return all posts" and leaves max_num_pages at 0 because pagination does not apply.

Closes #4334

How to test the Change

Changelog Entry

Fixed - max_num_pages becomes negative when posts_per_page is -1

Credits

Props @burhandodhy @dmitrijCraq

Checklist:

• I agree to follow this project's Code of Conduct.
• I have updated the documentation accordingly.
• I have added Critical Flows, Test Cases, and/or End-to-End Tests to cover my change.
• All new and existing tests pass.