chore: update model's size on import

[louis-jan]

Describe Your Changes

This PR updates the model's size on import as supported in the pull request. It includes minor refactoring of file size extension and consolidates features using the std::filesystem API.

Changes made

The provided git diff reflects several modifications across multiple files in the project. Here's a concise summary of these changes:

1. gguf_parser.cc:

   • Replaced manual file size determination with std::filesystem::file_size for reliability and simplicity.
   • Removed redundant code related to the old method of fetching the file size.

2. models.cc:

   • Introduced std::filesystem::file_size to fetch the file size when importing models, storing this size in the model_config.

3. download_service.cc and download_service.h:

   • Removed the custom GetLocalFileSize method and replaced its usage with std::filesystem::file_size for determining file sizes.
   • The download_service.h file had the method deleted from class specifications.
   • Adjustments were made to ensure curl_easy_setopt uses the new size-fetching method directly.

These changes simplify and modernize file size handling by leveraging the C++17 <filesystem> library.

[namchuai]

✅ thank you!