fix: skip main branch for cortexso model source if has metadata.yml

[vansangpfiev]

Describe Your Changes

This pull request includes changes to the ModelSourceService and huggingface_utils files to improve error handling and streamline the process of skipping certain branches. The most important changes are listed below:

Error Handling Improvements:

• engine/utils/huggingface_utils.h: Changed the return type of GetModelAuthorCortexsoHub from std::optional<std::string> to cpp::result<std::string, std::string> to better handle errors and provide more informative error messages.

Code Simplification:

• engine/services/model_source_service.cc: Updated the AddCortexsoRepo method to use the new cpp::result return type for GetModelAuthorCortexsoHub, ensuring that the author is correctly assigned and errors are handled properly.
• engine/services/model_source_service.cc: Added a check to skip the "main" branch if there are no errors with the model author, reducing unnecessary processing.

Fixes Issues

• Closes #
• Closes #

Self Checklist

• Added relevant comments, esp in complex areas
• Updated docs (for bug fixes / features)
• Created issues for follow-up changes or refactoring needed