Update foundational examples to use "user" role

Use system_instruction on LLM service constructors instead of adding
system messages to LLMContext. Messages added to context now use
"user" role.

examples/foundational/17-detect-user-idle.py

@@ -60,14 +60,14 @@ class IdleHandler:
         if self._retry_count == 1:
             # First attempt: Add a gentle prompt to the conversation
             message = {
-                "role": "system",
+                "role": "user",
                 "content": "The user has been quiet. Politely and briefly ask if they're still there.",
             }
             await aggregator.push_frame(LLMMessagesAppendFrame([message], run_llm=True))
         elif self._retry_count == 2:
             # Second attempt: More direct prompt
             message = {
-                "role": "system",
+                "role": "user",
                 "content": "The user is still inactive. Ask if they'd like to continue our conversation.",
             }
             await aggregator.push_frame(LLMMessagesAppendFrame([message], run_llm=True))
@@ -209,7 +209,7 @@ 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": "system", "content": "Please introduce yourself to the user."})
+        context.add_message({"role": "user", "content": "Please introduce yourself to the user."})
         await task.queue_frames([LLMRunFrame()])
         await asyncio.sleep(30)
         logger.info(f"Disabling idle detection")