refactor: use only GT for the test autoloader

[g105b]

The production GT\Async migration is already on master. Finish the phase-one autoload setup by removing the redundant Gt\Async\Test development mapping. Keep both production PSR-4 roots for existing consumers. Dependency namespace references are unchanged.

Validation on PHP 8.5.11:

• PHPUnit passed before and after the change. After: Tests: 36, Assertions: 155, Deprecations: 2.
• 14 fresh-process autoload checks passed across production classes/interfaces/traits, using both canonical and legacy roots.
• Composer validation passed (--no-check-publish --no-check-lock).
• Verified that edits introduce no changes to other PhpGt namespace references.
• git diff --check passed.
• Configured checks passed: phpstan, phpcs, phpmd.

[codacy-production]

Up to standards ✅

🟢 Issues 0 issues

Results:
0 new issues

View in Codacy

_{NEW Get contextual insights on your PRs based on Codacy's metrics, along with PR and Jira context, without leaving GitHub. Enable AI reviewer
TIP This summary will be updated as you push new changes.}