This PR adds resumable download functionality to the common_download_file_single function, allowing interrupted downloads to be resumed from where they left off instead of restarting from the beginning.

Problem

Previously, if a download was interrupted (due to network issues, process termination, etc.), the entire file would need to be re-downloaded from the start. This was particularly problematic for large model files that could take significant time to download.

Solution

The implementation adds HTTP Range request support to resume partial downloads:

Key Features

• Partial download detection: Automatically detects existing .downloadInProgress files and their current size
• Server compatibility check: Verifies server supports range requests via Accept-Ranges: bytes header
• HTTP Range requests: Uses Range: bytes=<start>- header to request only the remaining portion of the file
• Dual response handling: Correctly processes both HTTP 200 (full content) and HTTP 206 (partial content) responses
• Smart file handling: Opens files in append mode when resuming vs. create mode for new downloads

Example Behavior

common_download_file_single: found partial download: model.gguf.downloadInProgress (51200 bytes)
common_download_file_single: server supports range requests, resuming download from byte 51200
common_download_file_single: resuming download from https://example.com/model.gguf to /cache/model.gguf from byte 51200
common_download_file_single: successfully resumed partial download (HTTP 206)

Implementation Details

The changes are surgical and maintain full backward compatibility:

1. Before starting a download, check if a .downloadInProgress file exists
2. If found, send a HEAD request to verify the server supports range requests
3. If supported, add a Range: bytes=<offset>- header and open the file in append mode
4. Handle both 200 (server ignores range, sends full file) and 206 (partial content) responses
5. All existing functionality (ETag checking, authentication, retries) remains unchanged

Testing

Manually verified with a 124KB test file:

• Created partial download of 51,200 bytes
• Successfully resumed from byte 51,200 using HTTP Range request
• Received HTTP 206 response and completed full download
• Verified final file integrity

The implementation gracefully falls back to full downloads when servers don't support range requests, ensuring compatibility across different hosting environments.

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