Added GraphQL-spec error fields to OperationError (locations + extensions) and exhaustive optional-field coverage tests (#562)

Co-authored-by: xperiandri <2365592+xperiandri@users.noreply.github.com>
Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>

Author: Copilot