feat: abstract LLM provider architecture (replace callAnthropic)

[seba3c]

Summary

• introducing a new class CssAuditor with 3 main methods, audit, parse and export, now the API and cli use this object to call the different audit steps
• create an abstraction for llm providers API clients with its own config, this will make easier to add new providers later

[seba3c]

@nujovich ready for review

[nujovich]

@seba3c thanks! Have you tried different provider models ? E.g claude sonnet vs deepseekv4 ?

[seba3c]

@seba3c thanks! Have you tried different provider models ? E.g claude sonnet vs deepseekv4 ?

No, this will be next step, this is a previous refactor to make it easier to implement a new provider.