fix: rely on agent executor implementation for stream termination

Author: ishymko

src/a2a/server/request_handlers/default_request_handler_v2.py

@@ -271,11 +271,15 @@ async def on_message_send(  # noqa: D102
             ):
                 self._validate_task_id_match(task_id, event.id)
                 result = event
+                # DO break here as it's "return_immediately".
+                # AgentExecutor will continue to run in the background.
                 break
 
             if isinstance(event, Message):
                 result = event
-                break
+                # Do NOT break here as Message is supposed to be the only
+                # event in "Message-only" interaction.
+                # We rely on AgentExecutor here to simplify wrong implementations detection.
 
         if result is None:
             logger.debug('Missing result for task %s', request_context.task_id)
@@ -311,15 +315,14 @@ async def on_message_send_stream(  # noqa: D102
             request=request_context,
             include_initial_task=False,
         ):
+            # Do NOT break here as we rely on AgentExecutor to yield control
+            # to simplify wrong implementations detection.
             if isinstance(event, Task):
                 self._validate_task_id_match(task_id, event.id)
                 yield apply_history_length(event, params.configuration)
             else:
                 yield event
 
-            if isinstance(event, Message):
-                break
-
     @validate_request_params
     @validate(
         lambda self: self._agent_card.capabilities.push_notifications,