Fix ChatLiteLLM deprecation warning

Problem to solve

The class ChatLiteLLM was deprecated in LangChain 0.3.24 and will be removed in 1.0.

Step to reproduce

1. Enable warnings in Pytest.

   diff --git pyproject.toml pyproject.toml
   index 8068bf22..38b124b9 100644
   --- pyproject.toml
   +++ pyproject.toml
   @@ -129,7 +129,7 @@ ignore-words-list = "astroid"
   ignore-multiline-regex = "codespell:ignore-begin.*\n.*codespell:ignore-end"
   
   [tool.pytest.ini_options]
   -addopts = "--disable-warnings"
   +# addopts = "--disable-warnings"
   testpaths = ["tests"]
   filterwarnings = ["error::UserWarning"]
   asyncio_default_fixture_loop_scope = "function"

2. Run the test.

   ❯ poetry run pytest tests/prompts/test_base.py
   --snip--
   tests/prompts/test_base.py:320
     /Users/tle/code/gitlab/ai-gateway/tests/prompts/test_base.py:320: LangChainDeprecationWarning: The class `ChatLiteLLM` was deprecated in LangChain 0.3.24 and will be removed in 1.0. An updated version of the class exists in the :class:`~langchain-litellm package and should be used instead. To use it run `pip install -U :class:`~langchain-litellm` and import as `from :class:`~langchain_litellm import ChatLiteLLM``.
       ChatLiteLLM(
   

Proposal

Migrate to langchain-litellm library.

Further details

Links / references