Add default_providers configuration

[llenodo]

What this does

Screencast overview: https://www.loom.com/share/d044887130304d64b08eeab0eeee6f82

Adds a default_providers configuration option that routes models to specific providers by model ID prefix, eliminating the need to pass provider: on every call.

RubyLLM.configure do |config|
  config.default_providers = {
    'claude' => :bedrock,            # all claude models route to bedrock
    'claude-haiku' => :anthropic,    # except haiku, which goes direct to anthropic
    'gpt' => :azure,                 # all gpt models route to azure
    'gemini' => :vertexai,           # all gemini models route to vertex ai
    'claude-sonnet-4-5' => :openrouter,  # one specific model via openrouter
  }
end

Longer (more specific) keys take precedence. Explicit provider: arguments always override. Hooks into Models.resolve, so it applies to all model types (chat, embedding, image, transcription, moderation).

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

Required for new features

• I opened an issue before writing code and received maintainer approval
• Linked issue: [FEATURE] Add ability to set default providers #686

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)

AI-generated code

• I used AI tools to help write this code
• I have reviewed and understand all generated code (required if above is checked)

API changes

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