fix: handle missing Content-Length header in download_file

[bigcat88]

download_file() crashes with a KeyError when the server response doesn't include a Content-Length header. This happens with chunked transfer encoding, which is common when servers apply gzip compression or when downloading from certain CDNs.

The fix uses .get() instead of direct dict access. When Content-Length is missing, the progress bar falls back to an indeterminate style instead of crashing.

This is the proper fix for the issue described in #242. That PR worked around it by forcing Accept-Encoding: identity on all downloads, which would disable compression and slow things down for everyone. This fix handles the root cause in download_file itself so all callers benefit.

Closes #242

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

@@            Coverage Diff             @@
##             main     #371      +/-   ##
==========================================
+ Coverage   53.31%   53.36%   +0.05%     
==========================================
  Files          32       32              
  Lines        3588     3592       +4     
==========================================
+ Hits         1913     1917       +4     
  Misses       1675     1675              

Files with missing lines	Coverage Δ
comfy_cli/file_utils.py	79.87% <100.00%> (+0.50%)	⬆️

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