Add multisite (Network Admin) support for the admin page

[borislav-angelov]

Summary

Fixes #64.

The admin screen and the plugin action link were limited to the individual site's WP Admin. This expands both to also work in the Network Admin, which is important because on some multisite configurations regular administrators are not allowed to control plugins.

Changes

includes/Admin/Admin_Page.php

• add_hooks() now also registers:
  • network_admin_menu → add_and_initialize_network_page
  • the network_admin_plugin_action_links filter → filter_network_admin_plugin_action_links
• New add_network_page() / add_and_initialize_network_page() — registers the screen under Settings (settings.php) via add_submenu_page() with the manage_network_plugins capability, since the "Tools" menu does not exist in the Network Admin. The screen reuses the existing render_page / initialize_page callbacks, so it behaves the same as in the regular WP Admin.
• New filter_network_admin_plugin_action_links() — adds the action link in the Network Admin plugins list, pointing at network_admin_url( 'settings.php?page=plugin-check' ) (the page lives under settings.php here instead of tools.php).
• The shared link-building logic is extracted into a private add_check_action_link() helper; only the base page URL is parameterized, so the existing single-site URLs are unchanged.

Test coverage

tests/phpunit/tests/Admin/Admin_Page_Tests.php

• test_add_hooks extended to assert the two new Network Admin hooks are registered.
• test_add_and_initialize_network_page — ensures the admin page is added under settings.php using the Network Admin specific action (skipped on non-multisite).
• test_filter_network_admin_plugin_action_links — ensures the plugin link is added with the settings.php-based URL via the Network Admin specific filter.

Verified locally on WP 7.0 across both single-site and multisite runs — Admin_Page_Tests passes in both (the network page test is correctly skipped on single-site and executes on multisite). PHPCS and PHPStan are clean on the changed files.

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Co-authored-by: borislav-angelov <bangelov@git.wordpress.org>
Co-authored-by: eclarke1 <clarkeemily@git.wordpress.org>
Co-authored-by: vishalkakadiya <vishalkakadiya@git.wordpress.org>
Co-authored-by: davidperezgar <davidperez@git.wordpress.org>

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