Fix non streaming RAG problems (#607)

* Fix non-streaming failure in RAG services

* Fix non-streaming failure in API

* Fix agent non-streaming messaging

* Agent messaging unit & contract tests

trustgraph-flow/trustgraph/retrieval/graph_rag/rag.py

@@ -171,6 +171,7 @@ class Processor(FlowProcessor):
                 await flow("response").send(
                     GraphRagResponse(
                         response = response,
+                        end_of_stream = True,
                         error = None
                     ),
                     properties = {"id": id}