fix: support resume download for /pull

[vansangpfiev]

Describe Your Changes

This pull request introduces several improvements and new features to the download service in the engine. The most important changes include adding support for resuming downloads, enhancing logging for download progress, and refactoring the GetDownloadTask function.

Enhancements to download functionality:

• engine/services/download_service.cc: Added logic to check for existing files and resume downloads if the file is partially downloaded. This includes updating the file open mode to "ab" for appending and setting the CURLOPT_RESUME_FROM_LARGE option in SetUpCurlHandle to resume downloads. [1] [2] [3] [4]

Logging improvements:

• engine/services/download_service.cc: Enhanced logging to provide more detailed information about the download process, including the size of existing files and the number of bytes needed to complete the download.

Refactoring and code cleanup:

• engine/services/model_service.cc: Moved the GetDownloadTask function to a more appropriate location within the ModelService class and refactored it to include file size information for each download item. [1] [2]
• engine/services/download_service.h: Updated the SetUpCurlHandle function signature to include a resume_download parameter.
• engine/services/model_service.h: Added the GetDownloadTask function declaration to the ModelService class.

Minor fixes:

• engine/cli/utils/download_progress.cc: Fixed an off-by-one error in string truncation logic.
• engine/services/download_service.cc: Added handling for HTTP 206 (Partial Content) response code in ProcessCompletedTransfers.

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