Python: [A2A] Set message_id on AgentResponseUpdate for message-bearing paths (#6163)

Map A2A protocol message_id to AgentResponseUpdate.message_id in two paths
where it was previously omitted, aligning with .NET behavior:

1. Standalone A2AMessage: set message_id=msg.message_id (matches .NET
   ConvertToAgentResponseUpdate(Message) which sets both ResponseId and
   MessageId to message.MessageId)

2. TaskStatusUpdateEvent (terminal/input_required): set
   message_id=message.message_id (matches .NET which sets
   MessageId=statusUpdateEvent.Status.Message?.MessageId)

Fixes #5949

Co-authored-by: Copilot <copilot@github.com>
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

python/packages/a2a/agent_framework_a2a/_agent.py

@@ -492,6 +492,7 @@ class A2AAgent(AgentTelemetryLayer, BaseAgent):
                     contents=contents,
                     role="assistant" if msg.role == A2ARole.ROLE_AGENT else "user",
                     response_id=msg.message_id or str(uuid.uuid4()),
+                    message_id=msg.message_id,
                     additional_properties={"a2a_metadata": metadata} if metadata else None,
                     raw_representation=msg,
                 )
@@ -732,6 +733,7 @@ class A2AAgent(AgentTelemetryLayer, BaseAgent):
                 contents=contents,
                 role="assistant" if message.role == A2ARole.ROLE_AGENT else "user",
                 response_id=update_event.task_id,
+                message_id=message.message_id,
                 additional_properties={"a2a_metadata": merged_metadata} if merged_metadata else None,
                 raw_representation=update_event,
             )