Standardize models_anthropic() credentials handling to match other providers

[jcrodriguez1989]

Summary

Refactors models_claude() and models_anthropic() to use the same credential handling pattern as the other models_* functions (models_openai(), models_google_gemini(), etc.).

Changes

• Added credentials = NULL parameter to function signature
• Changed api_key parameter default from anthropic_key() to NULL
• Used as_credentials() helper for consistent credential resolution

[hadley]

This makes me wonder if it's worth extracting out credentials_anthropic() to reduce the duplication still further. (And similarly for for the other providers)