feat(llms): default values for default_llm_factories

[ludwiktrammer]

Currently values default values (i.e., values used when the user don't specify the option in their pyproject.toml) of each key in default_llm_factories is None. This means that our code can't assume that some default LLM will alway be provided, and have to either fallback to some other hardcoded LLM or fail.

Maybe we should provide some sensible defaults for those options.