Query: Do not replace the posts page with the front page when previewing.

[R1shabh-Gupta]

When a static front page and a separate posts page are configured, previewing the posts page incorrectly displays the front page.

WP_Query::parse_query() correctly identifies the request as both is_home and is_posts_page. However, in WP_Query::get_posts(), the front page preview logic only checks is_home, which is also true for the posts page. As a result, it replaces the requested page_id with page_on_front.

That behavior is needed when previewing the static front page, since its preview URL is a bare /?preview=true. This change avoids the substitution when the request explicitly targets the posts page.

Steps to reproduce

1. Create two pages, "My home" and "My posts".
2. Under Settings → Reading, set the homepage to "My home" and the posts page to
   "My posts".
3. Edit "My posts" in the block editor.
4. Click View → Preview in new tab.

Before this change the preview shows "My home". After it, "My posts".

Trac ticket: https://core.trac.wordpress.org/ticket/49969

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