DeepSeek v4.1 Flash is now available for internal beta testing

DeepSeek V4.1 Flash is now available for internal beta testing. It uses a new model architecture, with native multimodal support, stronger capabilities, faster speed, and lower cost.

Keep the `base_url` unchanged and set the model name to `deepseek-v4.1-flash-expires-on-0910` to use it. Pricing is currently the same as `deepseek-v4-flash`, with an account-level rate limit of 20 concurrent requests.