- Wired up `system_instruction` in `BaseOpenAILLMService`, `AnthropicLLMService`, and `AWSBedrockLLMService` so it works as a default system prompt, matching the behavior of the Google services. This enables sharing a single `LLMContext` across multiple LLM services, where each service provides its own system instruction independently.

  ```python
  llm = OpenAILLMService(
      api_key=os.getenv("OPENAI_API_KEY"),
      system_instruction="You are a helpful assistant.",
  )

  context = LLMContext()

  @transport.event_handler("on_client_connected")
  async def on_client_connected(transport, client):
      context.add_message({"role": "user", "content": "Please introduce yourself."})
      await task.queue_frames([LLMRunFrame()])
  ```
