[FEATURE] Support Anthropic's new structured outputs support

[tpaulshippy]

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?

with_schema works on some providers (OpenAI, gemini) but not on Anthropic because until now they have not had support for structured output.

Proposed solution

Implement the output_format key described at https://docs.claude.com/en/docs/build-with-claude/structured-outputs when users pass a schema to the render_payload method in lib/ruby_llm/providers/anthropic/chat.rb

Why this belongs in RubyLLM

Consistent support for structured outputs across providers.

[gusto-joshua]

This would be a great enhancement +1

[gusto-joshua]

@crmne would you be open to a PR for this?

[vojto]

@crmne I'd be happy to give it a try as well!

[vojto]

Here's my attempt: #528