Tests: Add unit test coverage for WP_Fatal_Error_Handler

[huzaifaalmesbah]

Description

Adds a test class for WP_Fatal_Error_Handler, which had no tests. The tests cover should_handle_error() for every PHP error type and the wp_should_handle_php_error filter, detect_error(), the early returns in handle() (no error, unhandled error, sandbox scraping, front end after headers are sent), and each message branch of display_default_error_template() with literal expected output, including the wp_php_error_message and wp_php_error_args filters.

This is a test-only change; no production code is modified.

Testing Instructions

1. Run the new tests on single site:
   npm run test:php -- --filter 'wpFatalErrorHandler'
2. Run them on multisite:
   npm run test:php -- --filter 'wpFatalErrorHandler' -c tests/phpunit/multisite.xml
3. Run them in random order:
   npm run test:php -- --filter 'wpFatalErrorHandler' --order-by=random
4. Run the whole group to confirm nothing else is affected:
   npm run test:php -- --group error-protection
5. Run PHPCS on the test file:
   node ./tools/local-env/scripts/docker.js run --rm php ./vendor/bin/phpcs tests/phpunit/tests/error-protection/wpFatalErrorHandler.php

Trac ticket: https://core.trac.wordpress.org/ticket/65819

Use of AI Tools

AI assistance: Yes
Tool(s): Claude Code
Model(s): Claude Opus 5.5
Used for: Brainstorming edge-case scenarios and drafting test boilerplate; test architecture, assertions, and local PHPUnit verification were written and validated by me.

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