fix(investigation): allow autonomous model baseline recovery

tests/unit/test_online_simulation.py

@@ -46,6 +46,17 @@ def test_online_simulation_rejects_invalid_step_before_calculating(monkeypatch):
     run.assert_not_called()
 
 
+def test_online_baseline_preserves_fractional_start_for_exact_impact_comparison(monkeypatch):
+    start = START.replace(microsecond=123456)
+    monkeypatch.setattr(simulation, "get_time", lambda _: {"HYDRAULIC TIMESTEP": "0:15:00"})
+    monkeypatch.setattr(simulation, "query_corresponding_element_id_and_query_id", lambda _: object())
+    run = Mock(return_value={"node_results": [{"node": "J1", "result": [{"pressure": 30}]}]})
+    monkeypatch.setattr(simulation, "run_simulation", run)
+    result = simulation.run_online_simulation("online", start, 60, output_node_ids=["J1"])
+    assert run.call_args.kwargs["modify_pattern_start_time"] == start.isoformat()
+    assert result["times"]["sample_times"] == [start.isoformat()]
+
+
 def test_mcp_online_simulation_does_not_copy_a_database(monkeypatch):
     from app.mcp.scenario_service import calculate_scenario