feat: 添加 OpenAI 插件

[he0119]

实现最简单的 assistant 功能。

[codecov]

Codecov Report

Attention: Patch coverage is 37.50000% with 40 lines in your changes missing coverage. Please review.

Project coverage is 80.49%. Comparing base (134b896) to head (d79860c).
Report is 1 commits behind head on main.

✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
src/plugins/openai/__init__.py	21.56%	40 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #639      +/-   ##
==========================================
- Coverage   81.59%   80.49%   -1.10%     
==========================================
  Files          64       66       +2     
  Lines        2505     2569      +64     
==========================================
+ Hits         2044     2068      +24     
- Misses        461      501      +40     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.