fix: resolve circular import between _data_pipeline and grid modules

The circular import was caused by:
- _data_pipeline.py importing Grid from timeseries.grid at module level
- grid.py importing connection_manager from microgrid
- microgrid/__init__.py importing _data_pipeline

Solution: Move Grid import from top-level to the grid() method where it's actually used, following the existing pattern in the codebase.

This fixes the CI failure in test_all_modules_importable.

Signed-off-by: Mathias L. Baumann <[email protected]>

Author: Marenz

src/frequenz/sdk/microgrid/_data_pipeline.py

@@ -26,9 +26,6 @@
 from ..actor._actor import Actor
 from ..timeseries import ResamplerConfig
 from ..timeseries._voltage_streamer import VoltageStreamer
-from ..timeseries.grid import Grid
-from ..timeseries.grid import get as get_grid
-from ..timeseries.grid import initialize as initialize_grid
 from ._data_sourcing import ComponentMetricRequest, DataSourcingActor
 from ._power_wrapper import PowerWrapper
 
@@ -47,6 +44,7 @@
     from ..timeseries.ev_charger_pool._ev_charger_pool_reference_store import (
         EVChargerPoolReferenceStore,
     )
+    from ..timeseries.grid import Grid
     from ..timeseries.logical_meter import LogicalMeter
     from ..timeseries.producer import Producer
     from ..timeseries.pv_pool import PVPool
@@ -206,6 +204,9 @@ def producer(self) -> Producer:
 
     def grid(self) -> Grid:
         """Return the grid measuring point."""
+        from ..timeseries.grid import get as get_grid
+        from ..timeseries.grid import initialize as initialize_grid
+
         if self._grid is None:
             initialize_grid(
                 channel_registry=self._channel_registry,