feat(stt): add FunASR / SenseVoice provider (#16473)

### Summary

Adds FunASR as a self-hosted speech-to-text provider through its
OpenAI-compatible `/v1/audio/transcriptions` endpoint.

This is a focused replacement for #15526 by @Rene0422 and relates to
#15448. The unrelated Markdown parser changes from the previous branch
are intentionally removed so this PR contains only the FunASR provider
integration.

- register FunASR as a `SPEECH2TEXT` factory;
- add `FunASRSeq2txt` with `sensevoice` and `http://localhost:8000/v1`
defaults, an optional API key, URL normalization, and inherited
transcription handling;
- wire FunASR into the current local-provider schema with a prefilled
local URL and official documentation link;
- discover the server's `/v1/models` dynamically and expose every
returned model as speech-to-text in the model picker;
- use RAGFlow's existing default provider icon fallback instead of
referencing a missing `funasr` asset;
- list FunASR in the supported-provider documentation;
- add focused backend and frontend regression tests.

### Validation

- focused backend pytest suite -> `7 passed`
- real CPU `funasr-server` + RAGFlow provider smoke test -> discovered
`fun-asr-nano`, `sensevoice`, and `paraformer`; transcribed a real WAV
as `我现在在录一段测试音频` (`10` tokens, `0.504s`)
- `ruff check` and `ruff format --check` on the changed Python files
- `python3 -m py_compile` on the provider and its test
- JSON parse and a semantic assertion for exactly one enabled FunASR
`SPEECH2TEXT` factory
- focused frontend Jest test -> `2 passed`
- ESLint and Prettier on all changed TypeScript files
- `npm run build` -> production build succeeded (`14,181` modules
transformed)
- `git diff --check`

### Deployment

Run FunASR separately and point the RAGFlow provider at it:

```bash
pip install funasr
funasr-server --device cuda --model sensevoice
```

The API key remains optional because the stock local server does not
require authentication. A key can still be supplied when the endpoint is
protected by a gateway.

---------

Signed-off-by: LauraGPT <LauraGPT@users.noreply.github.com>
Co-authored-by: LauraGPT <LauraGPT@users.noreply.github.com>

docs/guides/models/supported_models.mdx

@@ -28,6 +28,7 @@ A complete list of model providers supported by RAGFlow, which will continue to
 | Cohere                | `https://cohere.com`                            |
 | DeepSeek              | `https://www.deepseek.com`                      |
 | Fish Audio            | `https://fish.audio`                            |
+| FunASR                | `https://github.com/modelscope/FunASR`          |
 | FuturMix              | `https://futurmix.ai`                           |
 | Gemini                | `https://gemini.google.com`                     |
 | Google Cloud          | `https://cloud.google.com`                      |