feat: Add Avian as an LLM provider (#13256)

### What problem does this PR solve?

This PR adds [Avian](https://avian.io) as a new LLM provider to RAGFlow.
Avian provides an OpenAI-compatible API with competitive pricing,
offering access to models like DeepSeek V3.2, Kimi K2.5, GLM-5, and
MiniMax M2.5.

**Provider details:**
- API Base URL: `https://api.avian.io/v1`
- Auth: Bearer token via API key
- OpenAI-compatible (chat completions, streaming, function calling)
- Models:
  - `deepseek/deepseek-v3.2` — 164K context, $0.26/$0.38 per 1M tokens
  - `moonshotai/kimi-k2.5` — 131K context, $0.45/$2.20 per 1M tokens
  - `z-ai/glm-5` — 131K context, $0.30/$2.55 per 1M tokens
  - `minimax/minimax-m2.5` — 1M context, $0.30/$1.10 per 1M tokens

**Changes:**
- `rag/llm/chat_model.py` — Add `AvianChat` class extending `Base`
- `rag/llm/__init__.py` — Register in `SupportedLiteLLMProvider`,
`FACTORY_DEFAULT_BASE_URL`, `LITELLM_PROVIDER_PREFIX`
- `conf/llm_factories.json` — Add Avian factory with model definitions
- `web/src/constants/llm.ts` — Add to `LLMFactory` enum, `IconMap`,
`APIMapUrl`
- `web/src/components/svg-icon.tsx` — Register SVG icon
- `web/src/assets/svg/llm/avian.svg` — Provider icon
- `docs/references/supported_models.mdx` — Add to supported models table

This follows the same pattern as other OpenAI-compatible providers
(e.g., n1n #12680, TokenPony).

cc @KevinHuSh @JinHai-CN

### Type of change

- [x] New Feature (non-breaking change which adds functionality)
- [x] Documentation Update

docs/references/supported_models.mdx

@@ -18,6 +18,7 @@ A complete list of models supported by RAGFlow, which will continue to expand.
 | Provider              | LLM                | Image2Text         | Speech2text        | TTS                | Embedding          | Rerank             | OCR                |
 | --------------------- | ------------------ | ------------------ | ------------------ | ------------------ | ------------------ | ------------------ | ------------------ |
 | Anthropic             | :heavy_check_mark: |                    |                    |                    |                    |                    |                    |
+| Avian                 | :heavy_check_mark: |                    |                    |                    |                    |                    |                    |
 | Azure-OpenAI          | :heavy_check_mark: | :heavy_check_mark: | :heavy_check_mark: |                    | :heavy_check_mark: |                    |                    |
 | BaiChuan              | :heavy_check_mark: |                    |                    |                    | :heavy_check_mark: |                    |                    |
 | BaiduYiyan            | :heavy_check_mark: | :heavy_check_mark: |                    |                    | :heavy_check_mark: | :heavy_check_mark: |                    |