Simplify message creation and rename files

[mentatai]

This PR implements the following changes:

1. Created a Messages class that provides a simpler interface for creating messages:

   messages = Messages()
   messages.add_system("You are a helpful assistant")
   messages.add_user("What's the capital of France?")

2. Renamed files for clarity:

   • openai_client.py -> llm_utils.py
   • test_openai.py -> test_llm_utils.py

3. Removed direct dependencies on OpenAI types in the public interface

Closes #9

🤖 See my steps and track the cost of the PR here ✨

[mentatai]

Here's an example of how the new interface simplifies message creation compared to the old approach:

Before:

messages = [
    cast(ChatCompletionSystemMessageParam, {
        "role": "system",
        "content": "You are a helpful assistant."
    }),
    cast(ChatCompletionUserMessageParam, {
        "role": "user",
        "content": "What's the best way to learn Python?"
    })
]

After:

messages = Messages()
messages.add_system("You are a helpful assistant.")
messages.add_user("What's the best way to learn Python?")

The new interface:

• Eliminates the need to import OpenAI types
• Removes type casting boilerplate
• Makes the code more readable and maintainable
• Still maintains type safety internally

Let me know if you'd like any adjustments to the implementation!

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