Provides schema in anthropic system prompts

[jkogara]

What this does

• This PR will add a message to the system prompt of the native anthropic chat provider that instructs it to respond with the given schema if with_schema has called on the chat object.
• This is a change of behaviour as the schema is currently ignored by the the native anthropic provider.
• A minor change has also been made to parsing of responses when schema has been set, this was done to facilitate the anthropic models which often return json in code fences.

Type of change

• Bug fix
• New feature
• Breaking change
• Documentation
• Performance improvement

Scope check

• I read the Contributing Guide
• This aligns with RubyLLM's focus on LLM communication
• This isn't application-specific logic that belongs in user code
• This benefits most users, not just my specific use case

Quality check

• I ran overcommit --install and all hooks pass
• I tested my changes thoroughly
  • For provider changes: Re-recorded VCR cassettes with bundle exec rake vcr:record[provider_name]
  • All tests pass: bundle exec rspec
• I updated documentation if needed
• I didn't modify auto-generated files manually (models.json, aliases.json)

API changes

• Breaking change
• New public methods/classes
• Changed method signatures
• No API changes