Remove dependency on LLM API Key

[kamath]

Stagehand theoretically can run without AI (see e2e tests), so we shouldn't throw an error if an OPENAI_API_KEY or ANTHROPIC_API_KEY is not present, especially as we add new LLMClient implementations.

We should make model API keys optional, and throw an error on createChatCompletion (and wherever else an external model is actually used) if there's an invalid model API key.