fix: handle downloads without Content-Length

[dancingcowboy]

Summary

• use a fixed streaming block size when a response omits Content-Length
• report downloaded bytes when a percentage cannot be calculated
• add a regression test covering a response without the header

Without Content-Length, download_file previously used an uninitialized blocksize and assumed a known total while rendering progress. This can occur with chunked responses or intermediary proxies.

Validation

• python3 -m unittest test_utility.py
• python3 -m compileall -q .

[alplabin]

Hi @dancingcowboy,
I will close this PR as it is a duplicate of PR #488.