[FEATURE] Add `anthropic_api_base`

[ericproulx]

Scope check

• This is core LLM communication (not application logic)
• This benefits most users (not just my use case)
• This can't be solved in application code with current RubyLLM
• I read the Contributing Guide

Due diligence

• I searched existing issues
• I checked the documentation

What problem does this solve?

Set a base_url when using an Anthropic model. Enterprise user may pass through a LLM Gateway.

Proposed solution

Add anthropic_api_base config so that user can set a base_url. Can be set through ANTHROPIC_BASE_URL env. variable.

Why this belongs in RubyLLM

It's part of the provider configuration.

[ericproulx]

See #589

[nativestranger]

This allows users to point to custom endpoints such as proxies, custom deployments, or alternative Anthropic-compatible services.

This is really nice.