test: add async fetch coverage for TimeWindow data source (#869)

* test: add async fetch coverage for TimeWindow data source

* test: align TimeWindow async fetch coverage

---------

Co-authored-by: Nicholas Geneva <5533524+NickGeneva@users.noreply.github.com>

Author: LeSingh1

test/data/test_time_window.py

@@ -85,6 +85,10 @@ def __call__(self, time, variable):
             },
         )
 
+    async def fetch(self, time, variable):
+        """Mock async data fetch."""
+        return self(time, variable)
+
 
 class TestTimeWindowInitialization:
     """Test TimeWindow initialization and validation."""
@@ -357,6 +361,34 @@ def test_time_coordinate_alignment(self):
         assert len(result.time) == 1
         assert result.time.values[0] == np.datetime64(base_time)
 
+    @pytest.mark.asyncio
+    async def test_fetch_matches_sync_output_coordinates(self):
+        """Test async fetch applies offsets and preserves output coordinates."""
+        ds = MockDataSource()
+        tw = TimeWindow(
+            datasource=ds,
+            offsets=[timedelta(hours=-6), timedelta(hours=0), timedelta(hours=6)],
+            suffixes=["_tm1", "_t", "_tp1"],
+        )
+
+        base_time = datetime(2024, 1, 1, 12, 0)
+        result = await tw.fetch(base_time, ["t2m", "u10m"])
+
+        expected_vars = [
+            "t2m_tm1",
+            "t2m_t",
+            "t2m_tp1",
+            "u10m_tm1",
+            "u10m_t",
+            "u10m_tp1",
+        ]
+        actual_vars = [str(v) for v in result.coords["variable"].values]
+        assert actual_vars == expected_vars
+        assert result.time.values[0] == np.datetime64(base_time)
+        assert ds.call_history[0]["time"] == [datetime(2024, 1, 1, 6, 0)]  # -6h
+        assert ds.call_history[1]["time"] == [datetime(2024, 1, 1, 12, 0)]  # 0h
+        assert ds.call_history[2]["time"] == [datetime(2024, 1, 1, 18, 0)]  # +6h
+
 
 class TestTimeWindowErrorHandling:
     """Test error handling in TimeWindow."""