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/49b-thinking-google.py

@@ -104,7 +104,7 @@ 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": "Say hello briefly."})
+        context.add_message({"role": "developer", "content": "Say hello briefly."})
         # Replace the above with one of these example prompts to demonstrate
         # thinking.
         # These examples come from Gemini and Anthropic docs.