WP_Query: Apply LIMIT clause for singular queries with explicit posts_per_page

[Adi-ty]

This PR fixes a bug in WP_Query::get_posts() where the pagination logic incorrectly skips adding LIMIT clauses for singular queries, even when posts_per_page is explicitly set.

Trac ticket: https://core.trac.wordpress.org/ticket/63902

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This Pull Request is for code review only. Please keep all other discussion in the Trac ticket. Do not merge this Pull Request. See GitHub Pull Requests for Code Review in the Core Handbook for more details.

[github-actions]

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