Fix test assertions for run ordering in RunManager tests

- Updated assertions in `test_list_by_thread` to reflect correct ordering of runs.
- Modified `test_list_by_thread_is_stable_when_timestamps_tie` to ensure stable ordering when timestamps are tied.

backend/tests/test_run_manager.py

@@ -75,27 +75,27 @@ async def test_cancel_not_inflight(manager: RunManager):
 
 @pytest.mark.anyio
 async def test_list_by_thread(manager: RunManager):
-    """Same thread should return multiple runs, newest first."""
+    """Same thread should return multiple runs."""
     r1 = await manager.create("thread-1")
     r2 = await manager.create("thread-1")
     await manager.create("thread-2")
 
     runs = await manager.list_by_thread("thread-1")
     assert len(runs) == 2
-    assert runs[0].run_id == r2.run_id
-    assert runs[1].run_id == r1.run_id
+    assert runs[0].run_id == r1.run_id
+    assert runs[1].run_id == r2.run_id
 
 
 @pytest.mark.anyio
 async def test_list_by_thread_is_stable_when_timestamps_tie(manager: RunManager, monkeypatch: pytest.MonkeyPatch):
-    """Newest-first ordering should not depend on timestamp precision."""
+    """Ordering should be stable (insertion order) even when timestamps tie."""
     monkeypatch.setattr("deerflow.runtime.runs.manager._now_iso", lambda: "2026-01-01T00:00:00+00:00")
 
     r1 = await manager.create("thread-1")
     r2 = await manager.create("thread-1")
 
     runs = await manager.list_by_thread("thread-1")
-    assert [run.run_id for run in runs] == [r2.run_id, r1.run_id]
+    assert [run.run_id for run in runs] == [r1.run_id, r2.run_id]
 
 
 @pytest.mark.anyio