Expand OpenAI docs page (function calling, vision, embeddings, all models)

[AmitoVrito]

Expand docs/llms/openai.md from 35 lines to a full provider guide:

• All models with context windows and pricing table (gpt-4o, gpt-4o-mini, o1, o3, etc.)
• LLMConfig options specific to OpenAI (temperature, top_p, seed, response_format)
• Streaming example
• Function calling example with multiple tools
• Vision/multimodal with ImageContent
• Error handling (RateLimitError, ContextWindowError)
• Cost with TokenTracer/CostTracker
• Link to caching-retries.md