Administration: Guard network admin bootstrap on single sites

[ArkaPrabhaChowdhury]

Load the network administration bootstrap only after confirming that the request is running on a multisite installation.

Trac ticket: https://core.trac.wordpress.org/ticket/38076

Description

On single-site installations, wp-admin/network/admin.php previously loaded wp-admin/admin.php before the multisite guard ran. This allowed plugin-generated network-admin pages to be dispatched before the request was rejected.

This change:

• Moves the WordPress bootstrap before the network-admin dispatch path.
• Rejects network-admin requests on single-site installations before plugin-page dispatch.
• Adds regression coverage for the single-site bootstrap path.

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 could not be run locally because the test database was unavailable.
• The PR's CI follow-up addressed the shared DB_NAME already defined bootstrap failure; the current CI status should be reviewed separately.

Use of AI Tools

AI assistance: Yes
Tool(s): ChatGPT
Model(s): GPT-5
Used for: Reviewing the Trac ticket and current source, identifying the initial implementation and regression-test approach, assisting with the initial draft of the PR description, and helping diagnose the PHPUnit bootstrap failure. The final implementation, test changes, validation commands, and submitted description were reviewed and accepted by me.

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This Pull Request is for code review only. Please keep all other discussion in the Trac ticket. Do not merge this Pull Request. See GitHub Pull Requests for Code Review in the Core Handbook for more details.

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