Preload: parse post ID from p (path)

[ellatrix]

What?

Since #67199, post preload no longer works for this path:

/wp-admin/site-editor.php?canvas=edit&p=%2Fpage%2F<ID>

Why?

Preloading improves load performance when accessing site editor URLs directly.

How?

Parse the ID from the new path.

Testing Instructions

Load a page with the above URL and check the (first few) network requests.

Testing Instructions for Keyboard

Screenshots or screencast

[github-actions]

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Co-authored-by: ellatrix <[email protected]>
Co-authored-by: youknowriad <[email protected]>

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[youknowriad]

I think ideally, we should build some kind of "router" that has equivalent paths to the router we use in the site editor. But this address the important use-case.

[youknowriad]

Also would be cool to kind of allow using packages from packagist. Matching routes is a solved problem already in PHP in different packages.

[github-actions]

Flaky tests detected in fd482b9.
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/12117697666
📝 Reported issues:

• [Flaky Test] Redirection after template creation #60678 in /test/e2e/specs/site-editor/site-editor-url-navigation.spec.js
• [Flaky Test] submenu click on the arrow interactions #61884 in /test/e2e/specs/editor/blocks/navigation-frontend-interactivity.spec.js