download_strategy: fallback to Content-Range for file size

[shfc]

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Summary

• Extract file size from Content-Range header when Content-Length is missing
• Add tests for Content-Range header parsing

Why

Some file storage servers return Content-Range header instead of Content-Length in HEAD responses. This causes the download progress bar to not display the total file size during concurrent downloads.

Example:

$ curl -I 'https://cdn.crabnebula.app/asset/01KFBYACJVQEXTVPRRV97DCNPV'
# HTTP/2 200
# content-range: bytes 0-191050769/191050770
# content-disposition: attachment; filename="Conductor.app.tar.gz" 
# Note: no Content-Length header

Solution

Fallback to parsing total size from Content-Range header when Content-Length is missing or zero.

According to https://developer.mozilla.org/en-US/docs/Web/HTTP/Reference/Headers/Content-Range, the header has these formats:

• bytes <start>-<end>/<size> → extract <size>
• bytes */<size> (416 response) → extract <size>
• `bytes -/* (unknown size) → return nil

[MikeMcQuaid]

Makes sense, thanks and good idea!