fix: #3614875 Use parent::create() in TestEntityMappingWebformHandler

Ben Stallingsrequested to merge
issue/webform-3614875:3614875-testentitymappingwebformhandler.php-parameters-4 into 6.3.x

This was the only handler using new static(...) directly instead of parent::create(), which bypassed the standard container-based construction path.

Co-Authored-By: Claude Sonnet 5 noreply@anthropic.com

Closes #3614875

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