fix: add clarifying comment about done callback and consuming order (#154)

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Fixes #<issue_number_goes_here> 🦕

Author: kabir

sdk-server-common/src/main/java/io/a2a/server/requesthandlers/DefaultRequestHandler.java

@@ -162,6 +162,9 @@ public EventKind onMessageSend(MessageSendParams params) throws JSONRPCError {
         ResultAggregator.EventTypeAndInterrupt etai = null;
         try {
             EventConsumer consumer = new EventConsumer(queue);
+
+            // This callback must be added before we start consuming. Otherwise,
+            // any errors thrown by the producerRunnable are not picked up by the consumer
             producerRunnable.addDoneCallback(consumer.createAgentRunnableDoneCallback());
             etai = resultAggregator.consumeAndBreakOnInterrupt(consumer);
 
@@ -204,8 +207,9 @@ public Flow.Publisher<StreamingEventKind> onMessageSendStream(MessageSendParams
         try {
             EventConsumer consumer = new EventConsumer(queue);
 
+            // This callback must be added before we start consuming. Otherwise,
+            // any errors thrown by the producerRunnable are not picked up by the consumer
             producerRunnable.addDoneCallback(consumer.createAgentRunnableDoneCallback());
-
             Flow.Publisher<Event> results = resultAggregator.consumeAndEmit(consumer);
 
             Flow.Publisher<Event> eventPublisher =