Fix state bug in multipage navigation item retrieval

[RobjS]

This PR fixes a bug in the multipage navigation functionality by ensuring that repeated calls to MultipageInPageNavigation::getItems() return consistent results.

Before: getItems() would duplicate the items set if called more than once on the same instance. This could result in navigation rendering incorrectly.
Now: it always only returns the single correct set of items.

This PR was co-authored with CoPilot.

How to test

1. Spin up a site that uses the parent/child structure for long reads
2. Checkout the main branch of this plugin
3. Modify the LongReadPlugin\Navigation::getItems() method so it repeatedly gets the in-page items, e.g.

public static function getItems(): array
	{
		if (!isset(static::$chapterNavigation) || !isset(static::$inPageNavigation)) {
			return [];
		}
		$chapterNavItems = static::$chapterNavigation->getItems();
		$inPageNavItems = static::$inPageNavigation->getItems();
		$inPageNavItems = static::$inPageNavigation->getItems();
		$inPageNavItems = static::$inPageNavigation->getItems();
...

4. Confirm that this results in duplicate sets of in-page navigation items
5. Checkout this branch and make the same modification to that file
6. Confirm that navigation still renders correctly in the front-end, and no other changes are visible.

[sbrody]

Works locally