fix: bridge codex function tool calls back to openai clients

[typelicious]

Summary

• forward OpenAI-style tools and tool_choice to the ChatGPT Codex responses endpoint
• translate Codex function-call events back into OpenAI-compatible tool_calls for both non-stream and stream responses
• bump to v2.1.6 and cover the Codenomad-style tool path with provider regression tests

Validation

• rtk ruff check --no-cache faigate/providers.py tests/test_providers.py
• python3 -m pytest -q tests/test_providers.py
• local smoke against a temporary Gate instance with tools=[respira_get_site_context] returned finish_reason=tool_calls and a real tool_calls block on faigate/openai-codex-5.4-medium