Restored the default model to gpt-4.1 for OpenAI and Azure LLM services

The default model for OpenAILLMService and AzureLLMService was still set
to gpt-4o. Restored it to gpt-4.1. Also, removed hardcoded gpt-4o/gpt-4o-mini
model references from examples so they pick up the new default.

examples/foundational/04a-transports-daily.py

@@ -58,7 +58,6 @@ async def main():
         llm = OpenAILLMService(
             api_key=os.getenv("OPENAI_API_KEY"),
             settings=OpenAILLMService.Settings(
-                model="gpt-4o",
                 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.",
             ),
         )