[BFCL] Rename Misleading oss_model and proprietary_model Folders in model_handler Directory

[HuanzhiMao]

The folder names oss_model and proprietary_model within the model_handler directory should be updated to better reflect their actual purpose. proprietary_model should mean API-based inference while the other means local hosting-based inference.

The current naming is misleading, as there are open-source model handlers located in the proprietary_model folder, which causes confusion. Updating these names will improve clarity and maintain logical organization.