Administration: Guard network admin bootstrap on single sites

[ArkaPrabhaChowdhury]

Summary

• Load WordPress before the network administration bootstrap.
• Reject network-admin requests on single-site installs before plugin-page dispatch.
• Add regression coverage for the single-site bootstrap path.

Why

wp-admin/network/admin.php previously loaded wp-admin/admin.php before checking is_multisite(). That allowed plugin-generated network admin pages to be dispatched on single-site installations before the request was rejected.

Trac ticket: https://core.trac.wordpress.org/ticket/38076
Matching contribution issue: https://github.com/rtCamp/wp-contributions/issues/3744

Testing

• php -l src/wp-admin/network/admin.php
• php -l tests/phpunit/tests/admin/wpNetworkAdmin.php
• php vendor/bin/phpcbf --standard=phpcs.xml.dist src/wp-admin/network/admin.php tests/phpunit/tests/admin/wpNetworkAdmin.php
• php vendor/bin/phpcs --standard=phpcs.xml.dist src/wp-admin/network/admin.php tests/phpunit/tests/admin/wpNetworkAdmin.php
• PHPUnit is blocked locally because the test database is unavailable.

[ArkaPrabhaChowdhury]

Matching contribution issue: https://github.com/rtCamp/wp-contributions/issues/3744

[github-actions]

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