Close producers on flow stop to prevent stale non-persistent topics (#906)

Flow.stop() only stopped consumers, leaving response producers
connected to non-persistent Pulsar topics. After flow restart, the
orphaned producers held stale broker routing state, causing response
messages to never reach new consumers — manifesting as 120s timeouts
on document-embeddings and similar RPC paths.

Fix: Flow.stop() now explicitly stops all producers. Producer.stop()
closes the underlying Pulsar producer connection rather than just
setting a flag.

trustgraph-base/trustgraph/base/flow.py

@@ -34,6 +34,8 @@ class Flow:
     async def stop(self):
         for c in self.consumer.values():
             await c.stop()
+        for p in self.producer.values():
+            await p.stop()
         if self.librarian:
             await self.librarian.stop()
 

trustgraph-base/trustgraph/base/producer.py

@@ -34,6 +34,9 @@ class Producer:
 
     async def stop(self):
         self.running = False
+        if self.producer:
+            self.producer.close()
+            self.producer = None
 
     async def send(self, msg, properties={}):