PHPUnit workload fails when Composer platform_check references undefined STDERR

[chubes4]

Problem

A wordpress.phpunit workload fails before test discovery when a plugin's Composer autoloader includes the generated vendor/composer/platform_check.php in WP Codebox Playground.

Reproduction

From an Extra Chill plugin worktree:

homeboy review test extrachill-api --path /var/lib/datamachine/workspace/extrachill-api@fix-121-route-affinity

WP Codebox 0.12.14 installs Composer autoloaders successfully, then the PHPUnit workload aborts:

Recipe phase run_workloads failed: Recipe workflow steps[0] failed: wordpress.phpunit could not run: Error: Undefined constant \"STDERR\" at /wordpress/wp-content/plugins/extrachill-api/vendor/composer/platform_check.php:17

Run metadata:

• Homeboy run: 72c60725-ce63-4e50-9674-698de5c79457
• WP Codebox runtime: runtime-mrtlw45h-5gfo5q
• WP Codebox packages: 0.12.14
• Failure phase: run_workloads

Expected

The PHPUnit workload should provide CLI standard-stream constants or load Composer autoloaders in a way that does not make a generated platform check crash before test discovery.

Impact

Valid WordPress plugin tests cannot run in the sandbox whenever Composer generates a platform check that writes to STDERR on a failed requirement. The underlying platform-requirement message is also hidden by this secondary fatal error.