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/51-grok-realtime.py

@@ -211,7 +211,7 @@ Always be helpful and proactive in offering assistance.""",
 
     # Create context with initial message and tools
     context = LLMContext(
-        [{"role": "user", "content": "Say hello and introduce yourself!"}],
+        [{"role": "developer", "content": "Say hello and introduce yourself!"}],
         tools,
     )