feat: Add VolcEngine LLM provider support

- Add VolcEngine ProviderSpec entry in registry.py
- Add volcengine to ProvidersConfig class in schema.py
- Update model providers table in README.md
- Add description about VolcEngine coding plan endpoint

README.md

@@ -578,6 +578,7 @@ Config file: `~/.nanobot/config.json`
 > - **Groq** provides free voice transcription via Whisper. If configured, Telegram voice messages will be automatically transcribed.
 > - **Zhipu Coding Plan**: If you're on Zhipu's coding plan, set `"apiBase": "https://open.bigmodel.cn/api/coding/paas/v4"` in your zhipu provider config.
 > - **MiniMax (Mainland China)**: If your API key is from MiniMax's mainland China platform (minimaxi.com), set `"apiBase": "https://api.minimaxi.com/v1"` in your minimax provider config.
+> - **VolcEngine Coding Plan**: If you're on VolcEngine's coding plan, set `"apiBase": "https://ark.cn-beijing.volces.com/api/coding/v3"` in your volcengine provider config.
 
 | Provider | Purpose | Get API Key |
 |----------|---------|-------------|
@@ -591,6 +592,7 @@ Config file: `~/.nanobot/config.json`
 | `minimax` | LLM (MiniMax direct) | [platform.minimax.io](https://platform.minimax.io) |
 | `aihubmix` | LLM (API gateway, access to all models) | [aihubmix.com](https://aihubmix.com) |
 | `siliconflow` | LLM (SiliconFlow/硅基流动, API gateway) | [siliconflow.cn](https://siliconflow.cn) |
+| `volcengine` | LLM (VolcEngine/火山引擎) | [volcengine.com](https://www.volcengine.com) |
 | `dashscope` | LLM (Qwen) | [dashscope.console.aliyun.com](https://dashscope.console.aliyun.com) |
 | `moonshot` | LLM (Moonshot/Kimi) | [platform.moonshot.cn](https://platform.moonshot.cn) |
 | `zhipu` | LLM (Zhipu GLM) | [open.bigmodel.cn](https://open.bigmodel.cn) |