Consolidate provider branches in Chat#ruby_llm_context

[LasmarKhalifa]

The current case statement on Chat#ruby_llm_context has grown enough to the point where a refactor would be good. We need special cases for some providers that use different configurations (no api base url, extra variables, etc), but we don't need it on the ~80% of providers that use the same variables.

[LasmarKhalifa]

We added Anthropic, Perplexity and Gemini in this stack and with these ones we should be able to reduce the case statement to just checking for perplexity. We would also need the case statement if we are to add some of the providers that have a unique configuration, like Vertex AI (needs vertexai_project_id, vertexai_location, vertexai_service_account_key).