Support to LiteLLM

[matheper]

Integrate LiteLLM as an abstraction layer to standardize access to multiple LLM providers within the existing debug_gym.llms module.

Verify feasibility of:

• Support for OpenAI, Azure, Anthropic, HuggingFace, and Copilot
• Reducing boilerplate by unifying message formatting
• Ensuring compatibility with existing tokenize() and count_tokens() logic