[Model Request] Add support for Cartik/Sonexa-1-Chat

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Hello LMSYS and Arena team,

I would like to request adding our new open-source coding model to the Chatbot Arena leaderboard for evaluation. It is fully open and hosted on Hugging Face.

### Model Details:
* **Model Name:** Sonexa-1-Chat
* **Hugging Face Link:** https://huggingface.co/Cartik/Sonexa-1-Chat
* **Base Architecture:** Qwen3
* **Model Size:** 4B parameters
* **Primary Focus:** Code generation and technical instruction following

### Configuration & Templates:
* **Chat Template:** Standard Qwen3 / ChatML format (the model correctly utilizes `<|im_start|>` and `<|im_end|>` tokens).
* **Tokenizer:** Uses the default Qwen3Tokenizer setup.

The model has been tested and shows stable performance on standard programming tasks, outputting clean Markdown and syntactically correct code blocks. We would love to see how it performs in the blind testing environment against other models in its weight class.

Please let me know if you need any additional configurations, API endpoints, or if I should prepare a Pull Request for `fastchat/conversation.py` to ensure the template renders perfectly.

Thank you for your amazing work on the Arena!

Best regards,
Cartik

Start by reviewing fastchat/conversation.py and the Sonexa-1-Chat model details at its Hugging Face link, paying attention to the stated Qwen3/ChatML template and tokenizer configuration. Done means the model is added for Chatbot Arena evaluation and its template renders correctly in the blind testing environment.

Written by the indexing model from the issue text.