Add intermediate previews and graph reconnection

tests/test_service.py

@@ -277,7 +277,11 @@ class DataflowServiceTests(unittest.TestCase):
             self.session,
             tenant_id="tenant-1",
             actor_id="user-1",
-            payload=PipelinePreviewRequest(pipeline_id=pipeline.id, row_limit=1),
+            payload=PipelinePreviewRequest(
+                pipeline_id=pipeline.id,
+                preview_node_id="source",
+                row_limit=1,
+            ),
             principal=principal(),
         )
         self.session.commit()
@@ -287,6 +291,11 @@ class DataflowServiceTests(unittest.TestCase):
         self.assertEqual([{"id": 2, "amount": 15}], response.rows)
         self.assertEqual(2, response.total_rows)
         self.assertTrue(response.truncated)
+        self.assertIsNotNone(response.node_preview)
+        self.assertEqual("source", response.node_preview.node_id)
+        self.assertEqual([{"id": 1, "amount": 5}], response.node_preview.rows)
+        self.assertEqual(3, response.node_preview.total_rows)
+        self.assertTrue(response.node_preview.truncated)
         self.assertEqual(2, run.output_row_count)
         self.assertEqual(3, run.input_row_count)
         self.assertEqual(1, len(run.source_fingerprints))