feat: Add Google Gemini Provider Support

[ynbh]

description

adds gemini provider support via openai compatibility layer and fixes @tool decorator usage. includes test configuration updates for async support.

changes

• gemini provider: added GeminiModel with text generation, streaming, and tool calling support.
• tool decorator: made execute optional in @toolto support standard decorator usage.
• tests:
  • added tests for gemini provider and tool usage.
  • fixed test_tool_calling.py to correctly handle tool results.
  • added pytest.ini to enable asyncio_mode = "auto".
• style: formatting and lint fixes (ruff).

testing

set GEMINI_API_KEY and run:

pytest tests/test_gemini.py

let me know if there are fixes needed! ill try and update as needed. i couldn't test with with openai since I didn't have an API key, but all other tests pass.

[ynbh]

addresses #3