refactor: delete unused AsyncAiServiceClient, pass test diffs through repair path

Remove aiservice_async.py entirely — no longer imported anywhere after
the parallel evaluator was simplified to use ThreadPoolExecutor.

Replace string-based Failure values in parallel_evaluator with structured
EvalFailure dataclass that carries test diffs. The repair dispatcher in
function_optimizer now passes these diffs to the AI service, giving
repairs useful context about what failed.

Author: KRRT7