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/55zm-update-settings-gemini-live-vertex.py

@@ -53,16 +53,10 @@ async def run_bot(transport: BaseTransport, runner_args: RunnerArguments):
         credentials=os.getenv("GOOGLE_VERTEX_TEST_CREDENTIALS"),
         project_id=os.getenv("GOOGLE_CLOUD_PROJECT_ID"),
         location=os.getenv("GOOGLE_CLOUD_LOCATION"),
+        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)
 
     pipeline = Pipeline(