Plugins: Distinguish a missing admin page from an access denial (#14060)

[roshniahuja]

Description

Accessing the URL of a plugin admin page that is no longer registered previously
showed the misleading "Sorry, you are not allowed to access this page." message,
implying a permissions problem where none exists.

This introduces admin_page_exists(), which determines whether the requested page
is registered independently of the current user's capabilities. When the page does
not exist, wp_die() is now called with a clearer "The requested page does not
exist." message and a 404 status, and a new admin_page_not_found action fires,
mirroring the existing admin_page_access_denied action.
user_can_access_admin_page() reuses the new function so the registration check is
no longer duplicated.

Testing Instructions

• Install and activate a plugin that includes a settings page.
• Navigate to the plugin's settings page and copy the URL.
• Deactivate the plugin.
• Paste the copied URL into the browser's address bar and press Enter.
• You should see "The requested page does not exist." (404) instead of "Sorry, you
  are not allowed to access this page."

Unit tests:
npm run test:php -- --filter admin_page_exists tests/phpunit/tests/admin/includesPlugin.php

Trac ticket: https://core.trac.wordpress.org/ticket/14060

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