Fix: Disable post cache for cross-site queries

[burhandodhy]

Description of the Change

On multisite, cross-site Elasticsearch queries (site__in, site__not_in, or ep_search_scope → all) could prime the blog-specific posts cache with posts from other sites. Because that cache is keyed by post ID only, a later get_post() for a local ID could return another site's content.

This change disables cache_results for those cross-site query scopes in get_es_posts(), after the final scope is resolved.

Closes #4352

How to test the Change

Changelog Entry

Changed - Set cache_results to false if the query args contain site__in or site__not_in, or if ep_search_scope is set to all.

Credits

Props @burhandodhy @dannyreaktiv

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.