Navigation Link: Use dynamic URL and label at render time and sync label in editor

[dhruvikpatel18]

What?

Closes #38253
This PR fixes the core/navigation-link block so that navigation menu items always reflect the current permalink and title of the linked post, page, or taxonomy term, both on the frontend and in the Site Editor.

Why?

When a navigation menu item is added via the Site Editor, its url and label are stored as block attributes at save time. If a post's title or slug is later changed, the menu item continued to show the old URL and label, with no automatic update. This was reported in #38253.

Even when WordPress could redirect the old URL, it is better to output the correct link from the start so it works without relying on redirects.

Testing Instructions

1. Create a new Page titled "Test Page" with slug test-page and publish it.
2. Go to Appearance → Editor → Navigation and add "Test Page" as a navigation menu item.
3. Save the navigation menu.
4. Go back and edit the page: change its title to "My Changed Page" and its slug to my-changed-page. Update/publish.
5. Confirm the changed page title in frontend and site editor as well.

Screenshots or screencast

Screen.Recording.2026-07-03.at.4.01.12.PM.mov

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