Expand PROVIDERS.md with complete implementation guide
Add comprehensive documentation for new provider implementers:
- Full generate()/agenerate() function signature
- Parameter details table with provider-specific nuances (vision, thinking, timeouts)
- Token usage format examples for OpenAI, Anthropic, Google
- Finish event format with usage field
- Batch processing note (automatic via agenerate)
- OpenAI-compatible providers section for DO/Azure/local LLMs
- Expanded 15-item checklist including get_model_provider(), fixtures, and doc updates
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>