Add typed Arrow execution backend boundary

src/govoplan_dataflow/backend/schema_validation.py

@@ -55,6 +55,15 @@ def validate_graph_schemas(
     *,
     ordered: list[str],
 ) -> list[DataflowDiagnostic]:
+    _, diagnostics = propagate_graph_schemas(graph, ordered=ordered)
+    return diagnostics
+
+
+def propagate_graph_schemas(
+    graph: PipelineGraph,
+    *,
+    ordered: list[str],
+) -> tuple[dict[str, SchemaState], list[DataflowDiagnostic]]:
     node_by_id = {node.id: node for node in graph.nodes}
     inputs = _graph_inputs_by_port(graph)
     schemas: dict[str, SchemaState] = {}
@@ -81,7 +90,7 @@ def validate_graph_schemas(
             )
         schemas[node.id] = result.state
         diagnostics.extend(result.diagnostics)
-    return diagnostics
+    return schemas, diagnostics
 
 
 def _graph_inputs_by_port(