test: validate concurrency acceptance reason in async E2E test

Ensure the async E2E test verifies that optimizations are accepted
for the correct reason (concurrency improvement).

Author: KRRT7

tests/scripts/end_to_end_test_async.py

@@ -8,6 +8,7 @@ def run_test(expected_improvement_pct: int) -> bool:
     config = TestConfig(
         file_path="main.py",
         min_improvement_x=0.1,
+        expected_acceptance_reason="concurrency",
         coverage_expectations=[
             CoverageExpectation(
                 function_name="retry_with_backoff",