Query Block: Fix 'parents' argument validation

[Mamaduka]

What?

Part of #68620

PR updates Query parents argument validation for hierarchical post types to allow querying only top-level entries, using "parents":[0].

Why?

• A zero is a valid parent ID for hierarchical post types. Used to display top-level items.
• Helps developers provide custom query patterns that only display top-level pages.

Testing Instructions

1. Ensure you have a couple of pages with and without parents.
2. Create a page.
3. Add a Test Query using the code editor.
4. Confirm that the Query block displays only the top-level items.

Testing Instructions for Keyboard

Same.

Test Query

Markup

<!-- wp:query {"queryId":1,"query":{"perPage":10,"pages":0,"offset":0,"postType":"page","order":"desc","orderBy":"date","author":"","search":"","exclude":[],"sticky":"","inherit":false,"parents":[0],"format":[]}} -->
<div class="wp-block-query"><!-- wp:post-template -->
<!-- wp:post-title /-->

<!-- wp:post-date /-->
<!-- /wp:post-template -->

<!-- wp:query-pagination -->
<!-- wp:query-pagination-previous /-->

<!-- wp:query-pagination-numbers /-->

<!-- wp:query-pagination-next /-->
<!-- /wp:query-pagination -->

<!-- wp:query-no-results -->
<!-- wp:paragraph {"placeholder":"Add text or blocks that will display when a query returns no results."} -->
<p></p>
<!-- /wp:paragraph -->
<!-- /wp:query-no-results --></div>
<!-- /wp:query -->

[github-actions]

Flaky tests detected in ae0aa39.
Some tests passed with failed attempts. The failures may not be related to this commit but are still reported for visibility. See the documentation for more information.

🔍 Workflow run URL: https://github.com/WordPress/gutenberg/actions/runs/13071058461
📝 Reported issues:

• [Flaky Test] should show the original content after revert and page reload #68604 in /test/e2e/specs/site-editor/template-revert.spec.js