Use "developer" role for programmatic conversation-kickoff messages

These messages are developer instructions to the assistant (e.g. "Please
introduce yourself to the user"), not simulated user input. The
"developer" role is semantically correct for this purpose.

examples/foundational/20e-persistent-context-aws-nova-sonic.py

@@ -264,7 +264,9 @@ async def run_bot(transport: BaseTransport, runner_args: RunnerArguments):
     async def on_client_connected(transport, client):
         logger.info(f"Client connected")
         # Kick off the conversation.
-        context.add_message({"role": "user", "content": "Please introduce yourself to the user."})
+        context.add_message(
+            {"role": "developer", "content": "Please introduce yourself to the user."}
+        )
         await task.queue_frames([LLMRunFrame()])
         # HACK: if using the older Nova Sonic (pre-2) model, you need this special way of
         # triggering the first assistant response. Note that this trigger requires a special