Add configurable Anthropic API base URL

[ericproulx]

Summary

Adds support for configurable Anthropic API base URL through a new anthropic_api_base configuration option. This allows users to point to custom endpoints such as proxies, custom deployments, or alternative Anthropic-compatible services.

Changes

• Configuration: Added anthropic_api_base configuration attribute
• Provider: Modified Anthropic provider to use custom base URL when configured, with fallback to default https://api.anthropic.com
• URL Fixes: Removed leading slashes from API endpoint paths in chat and models modules for proper URL construction
• Console: Updated bin/console to support ANTHROPIC_BASE_URL environment variable
• Tests: Added comprehensive test coverage for the new configuration option

Use Cases

This enhancement enables:

• Using proxy servers for Anthropic API requests
• Custom deployments of Anthropic-compatible services
• Testing against mock servers
• Regional or enterprise-specific endpoints

Configuration Example

RubyLLM.configure do |config|
  config.anthropic_api_key = "your-api-key"
  config.anthropic_api_base = "https://custom-anthropic-endpoint.example.com"
end

Or via environment variable:

export ANTHROPIC_BASE_URL="https://custom-anthropic-endpoint.example.com"

Backward Compatibility

✅ Fully backward compatible - defaults to official Anthropic API endpoint when not configured