Connect to Siliconflow reliably with the corrected default base URL

Updated the default base_url for the Siliconflow model provider from .com to .cn to match Siliconflow's actual API endpoint. Requests were previously routed to an incorrect domain, causing connection failures for users relying on the default configuration.

Details**:**

• Corrects the default base_url to siliconflow.cn
• Users who had already overridden base_url explicitly are unaffected
• No other configuration changes required