feat: fallback to post type slug for parent page

[ictbeheer]

For the overwhelming majority of post types the parent page slug is equal to the post type slug.
This PR still allows a different slug, but introduces a fallback to the post type slug if no parent-page slug is configured.

See also Brave Components

[Copilot]

Pull request overview

Adds a fallback mechanism for resolving a “parent page” by using the current post type’s rewrite slug when an explicit parent-page support slug is not configured.

Changes:

• Extracts $postType and $supports into variables for reuse/readability.
• Falls back to get_post_type_object($postType)->rewrite['slug'] when parent-page support does not provide a slug.

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[Copilot]

get_post_type_object($postType)->rewrite['slug'] can throw warnings/fatal errors if get_post_type($postId) returns false (invalid post) or if get_post_type_object() returns null, and rewrite can be false (non-rewritten CPT) so array access may warn. Consider guarding for a falsy $postType, storing the post type object in a variable, and only reading $obj->rewrite['slug'] when the object exists and rewrite is an array with a slug key (otherwise fall back to null).

Suggested change

	$supports = get_all_post_type_supports($postType);
	$parentPageSlug = $supports['parent-page'][0]['slug'] ?? get_post_type_object($postType)->rewrite['slug'] ?? null;
	if (! $postType) {
	return [];
	}
	$supports = get_all_post_type_supports($postType);
	$postTypeObject = get_post_type_object($postType);
	$rewriteSlug = is_object($postTypeObject) && is_array($postTypeObject->rewrite) && isset($postTypeObject->rewrite['slug'])
	? $postTypeObject->rewrite['slug']
	: null;
	$parentPageSlug = $supports['parent-page'][0]['slug'] ?? $rewriteSlug;