65655 functions wp die

[pbearne]

This PR introduces dedicated unit tests for the wp_die() function and its primary HTML handler _default_wp_die_handler() in wp-includes/functions.php.

Following the project's testing guidelines, the tests are split into two files:
•tests/phpunit/tests/functions/wpDie.php: Focuses on handler selection logic (Standard vs. Ajax), handling of WP_Error objects, and ensuring the correct callback is triggered via filters.
•tests/phpunit/tests/functions/_defaultWpDieHandler.php: Verifies the HTML output generated by the default handler, including the error message, title, additional error lists, and action links.

The tests use WPDieException to intercept execution termination and MockAction to verify filter interactions.

Trac ticket: https://core.trac.wordpress.org/ticket/65655

AI Disclosure:
•AI assistance: Yes
•Tool(s): Junie (JetBrains)
•Model(s): gemini-3-flash-preview
•Used for: Code analysis, test implementation, and workflow management.

[github-actions]

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Props pbearne, mukesh27.

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[github-actions]

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