Modern tool calling support

New Features:

1. Added modern tool-calling support to ChatSession

   • Supports tools, tool_choice, and parallel_tool_calls
   • Tool results can be sent back with role = "tool" and tool_call_id

2. Added per-request parameter overrides for session methods

   • chat:send(...) and chat:generate_response(...) now accept request-specific overrides
   • ResponsesChatSession:send(...) now supports the same pattern

3. Added new tool-calling examples

   • New chat completions example in examples/chat_completions/tools.lua
   • Updated Responses example to show a full tool-call round trip

Improvements:

1. Improved ResponsesChatSession parity with ChatSession

   • Better support for tool-call round trips using function_call_output
   • Session usage and examples now align more closely across both APIs

2. Improved streaming tool-call handling

   • Streaming tool-call deltas are aggregated into complete assistant messages

3. Expanded documentation and test coverage

   • README now documents tool calling and per-request overrides
   • Added tests for tool calls, streaming aggregation, and session overrides