Responses API, Gemini compatibility, simplified streaming

Breaking Changes:

1. ChatSession no longer exported from main module

   • Use require("openai.chat_completions").ChatSession instead

2. Default model changed from gpt-3.5-turbo to gpt-4.1

3. chat() streaming callback now receives (delta, raw) instead of (chunk)

   • Use create_chat_completion() for the previous raw chunk behavior

New Features:

1. Added Responses API support

   • create_response(params): Create a new response
   • response(response_id): Get a response by ID
   • delete_response(response_id): Delete a response
   • cancel_response(response_id): Cancel a response

2. Added ResponsesChatSession for conversational state with Responses API

3. Added Gemini compatibility client

   • require("openai.compat.gemini") for Google Gemini API

4. Added create_chat_completion() method for raw streaming behavior

Improvements:

1. SSE parsing moved to dedicated openai.sse module

2. Removed lpeg dependency in favor of simpler line-based SSE parsing