Upgrade/Install: Check for wp-config.php before wp-config-sample.php

[ritakikani]

This moves the wp-config-sample.php existence check after the existing
wp-config.php checks in wp-admin/setup-config.php.

This avoids checking for wp-config-sample.php when wp-config.php already
exists, including environments where open_basedir restrictions may trigger
a warning while checking for the sample file.

Testing

Tested manually with wp-config.php present and wp-config-sample.php
temporarily unavailable.

setup-config.php correctly detected the existing wp-config.php before
attempting to check wp-config-sample.php.

Additional checks performed:

• php -l src/wp-admin/setup-config.php
• vendor/bin/phpcs src/wp-admin/setup-config.php
• git diff --check

Trac ticket: https://core.trac.wordpress.org/ticket/53298

Use of AI Tools

AI assistance: Yes
Tool(s): ChatGPT
Model(s): GPT-5.6 Sol
Used for: Guidance through the WordPress Core contribution workflow, understanding the ticket, testing steps, Git workflow, and PR description. The code change was reviewed and tested locally 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.

[github-actions]

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Props kikanirita.

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[github-actions]

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