Apply block styles to Page List items when nested in a Submenu

[sarthaknagoshe2002]

What?

Closes #80000

This PR ensures that custom styles (like text color, background color, font size, etc.) applied to a Page List block are rendered on the frontend when the block is nested inside a Navigation Submenu.

Why?

When a Page List is placed inside a Submenu, its outer <ul> wrapper is intentionally omitted to prevent invalid HTML nesting (<ul> inside <ul>). However, because get_block_wrapper_attributes() was tied to that wrapper, all block support classes and inline styles were being silently discarded on the frontend, resulting in a mismatch between the editor (where styles worked) and the published site (where they disappeared).

How?

This fix intercepts the generated wrapper attributes when the block is nested. It extracts the class and style attributes from the discarded wrapper and manually injects them into the top-level <li> elements ($depth === 0) generated by block_core_page_list_render_nested_page_list().

Note on theme spacing: Themes that add intentional spacing between submenu items (like Twenty Twenty-Five's margin-bottom: 3px for keyboard focus outline clearance) will show a thin unstyled gap between colored Page List items. This PR intentionally leaves that gap alone rather than injecting a hardcoded CSS height/margin hack, preserving the theme's box-model and accessibility choices.

This can be fixed specifically for Twenty Twenty-Five theme using:

.wp-block-navigation {
    .wp-block-navigation-submenu .wp-block-navigation-item.wp-block-page-list:not(:last-child) {
		position: relative;

		&::after {
			content: "";
			position: absolute;
			inset-inline: 0;
			inset-block-end: -3px;
			block-size: 3px;
			background-color: inherit;
			pointer-events: none;
		}
	}
}

Testing Instructions

1. Go to Appearance → Editor or create a new Post/Page.
2. Insert a Navigation block.
3. Add a Submenu block inside the Navigation block.
4. Insert a Page List block inside the Submenu. (Ensure you have a few published pages on your site so they populate).
5. Select the nested Page List block and use the Styles sidebar to apply a custom Text color and Background color.
6. Confirm the styles apply correctly in the Editor.
7. Save and view the site on the frontend.
8. Open the submenu and verify that the text and background colors are now correctly applied to the Page List items.

Screenshots or screencast

Before	After

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• Labels found: [Package] Block library, [Block] Submenu.

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Co-authored-by: sarthaknagoshe2002 <sarthaknagoshe2002@git.wordpress.org>
Co-authored-by: prachigarg19 <prachigarg19@git.wordpress.org>
Co-authored-by: saulirajala <saulirajala@git.wordpress.org>

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