Fix PHPUnit extension test discovery

[lennartvava]

Summary

Fix PHPUnit configuration generation for Drupal extensions so test and source discovery no longer recursively scans Composer dependencies.

Changes

• Restrict extension test-suite discovery to project tests/ and optional modules/ paths.
• Restrict coverage source discovery to project src/ and optional modules/ paths.
• Prevent PHPUnit from traversing vendor/, which could cause test runs to stall or exceed CI timeouts.
• Add regression coverage for the generated extension PHPUnit configuration.
• Add the missing 5.0.1 changelog entry and document the 5.0.2 fix.

Verification

• composer validate --strict
• vendor/bin/phpunit --configuration=phpunit.xml.dist tests/src/Unit/GrumPHP/ConfigFileMergerTest.php
• git diff --check