Return session name in runner response (#6112)

Makes it easier to then run evals w/o thinking about specifying a
session

tests/unit_tests/client/test_runner_utils.py

@@ -201,4 +201,4 @@ async def test_arun_on_dataset(monkeypatch: pytest.MonkeyPatch) -> None:
             ]
             for uuid_ in uuids
         }
-        assert results == expected
+        assert results["results"] == expected