examples(foundational): use system_instruction in all examples

examples/foundational/21-tavus-transport.py

@@ -54,16 +54,12 @@ async def main():
             voice_id="a167e0f3-df7e-4d52-a9c3-f949145efdab",
         )
 
-        llm = GoogleLLMService(api_key=os.getenv("GOOGLE_API_KEY"))
+        llm = GoogleLLMService(
+            api_key=os.getenv("GOOGLE_API_KEY"),
+            system_instruction="You are a helpful LLM in a WebRTC call. Your goal is to demonstrate your capabilities in a succinct way. Your output will be spoken aloud, so avoid special characters that can't easily be spoken, such as emojis or bullet points. Respond to what the user said in a creative and helpful way.",
+        )
 
-        messages = [
-            {
-                "role": "system",
-                "content": "You are a helpful LLM in a WebRTC call. Your goal is to demonstrate your capabilities in a succinct way. Your output will be spoken aloud, so avoid special characters that can't easily be spoken, such as emojis or bullet points. Respond to what the user said in a creative and helpful way.",
-            },
-        ]
-
-        context = LLMContext(messages)
+        context = LLMContext()
         user_aggregator, assistant_aggregator = LLMContextAggregatorPair(
             context,
             user_params=LLMUserAggregatorParams(vad_analyzer=SileroVADAnalyzer()),
@@ -95,7 +91,7 @@ async def main():
         async def on_client_connected(transport, participant):
             logger.info(f"Client connected")
             # Kick off the conversation.
-            messages.append(
+            context.add_message(
                 {
                     "role": "system",
                     "content": "Start by greeting the user and ask how you can help.",