feat: Support Opencode Zen free models

[tan-yong-sheng]

As request, I would like to request for opencode zen free models like https://github.com/jasonxu114514/opencode2api , thanks a lot for your consideration

As a temporary workaround, I am using #6018 and then put that api endpoint into CLIProxyAPI

[bilogic]

https://github.com/jasonxu114514/opencode2api doesn't work?

[tan-yong-sheng]

@bilogic I am using https://github.com/mannnrachman/pi-bansos as well (without api key), and compare with opencode zen in opencode2api, the thinking text seems not handled properly or rendered properly in pi agent (not that sure) and I feel using opencode2api (with opencode zen api key) seems much dumber (e.g., keep looping and stuck) than the without api keys even using the same model ... That's why I am wondering if it is the proxy problem or the opencode provider problem ...