fix: Update imports to use engine.pipeline instead of engine.pipeline.core

The old engine/pipeline/core.py file was removed as part of the Sideline/Mainline split.
All imports that referenced engine.pipeline.core have been updated to use engine.pipeline
which re-exports from sideline.pipeline.core.

This ensures consistency and avoids duplicate DataType enum instances.

engine/data_sources/pipeline_introspection.py

@@ -67,13 +67,13 @@ class PipelineIntrospectionSource(DataSource):
 
     @property
     def inlet_types(self) -> set:
-        from engine.pipeline.core import DataType
+        from engine.pipeline import DataType
 
         return {DataType.NONE}
 
     @property
     def outlet_types(self) -> set:
-        from engine.pipeline.core import DataType
+        from engine.pipeline import DataType
 
         return {DataType.SOURCE_ITEMS}