📝 Add docstrings to `nanogpt-provider`

[coderabbitai[bot]]

Docstrings generation was requested by @0xGingi.

• https://github.com/wavetermdev/waveterm/pull/2746#issuecomment-3707135396

The following files were modified:

• cmd/testai/main-testai.go
• pkg/aiusechat/usechat-mode.go

These file types are not supported

• docs/docs/waveai-modes.mdx
• schema/waveai.json

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