[Docs]: Add JSDoc comments to lib/providers.ts for better DX

[ixchio]

📚 Documentation Enhancement

Problem

The lib/providers.ts file contains important configuration for AI providers and models, but lacks JSDoc documentation. This makes it harder for new contributors to understand the codebase.

Proposed Solution

Add comprehensive JSDoc comments to:

1. Type definitions:

   • ProviderType - explain what providers are supported
   • Provider interface - document each field
   • AIModel interface - document each field including optional flags

2. Constants:

   • FREE_PROVIDERS - explain the provider configuration
   • FREE_MODELS - document the model selection rationale

3. Helper functions:

   • getProviderById()
   • getModelById()
   • getModelsByProvider()
   • getFreeModels()
   • isProviderFree()

Acceptance Criteria

• All exported types have JSDoc comments
• All exported functions have JSDoc comments with @param and @return
• Examples are provided where helpful
• Comments follow TypeScript best practices

---

Estimated time: 30-45 minutes
Difficulty: Easy ⭐
Perfect for first-time contributors!