fix: #3593394 Add PHP executable variants to insecure extensions

Tara Wijayagunawardanarequested to merge
issue/drupal-3593394:3593394-expand-insecure-extensions into main

Closes #3593394

  • Adds PHP executable extension variants (php3, php4, php5, php7, php8, and pht) to FileSystemInterface::INSECURE_EXTENSIONS and the matching insecure extension regex.
  • Adds test coverage for filename sanitization and JSON:API upload rejection.

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