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/33-gemini-rag.py

@@ -248,7 +248,9 @@ Your response will be turned into speech so use only simple words and punctuatio
     async def on_client_connected(transport, client):
         logger.info(f"Client connected")
         # Start conversation - empty prompt to let LLM follow system instructions
-        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()])
 
     @transport.event_handler("on_client_disconnected")