fix naturalsize ValueError for custom format strings with text (fixes #366)

[aDragon0707]

Fixes #366.

naturalsize() raised ValueError for custom format strings with surrounding text (e.g. format="Size: %.1f"), because the rollover check passed the full formatted result to float().

• The rollover step now tries to parse the formatted mantissa as a float and skips the rollover when the formatted result is not a plain number, preserving the pre-4.16 behavior for text formats.
• Plain numeric formats keep the rollover behavior (999_999 -> "1.0 MB" still works).

Regression test added (test_naturalsize_custom_format_with_text): prefix and suffix cases.

tests/test_filesize.py → 77 passed, 0 failed.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.57%. Comparing base (3201e70) to head (93e7b85).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #396   +/-   ##
=======================================
  Coverage   99.56%   99.57%           
=======================================
  Files          12       12           
  Lines         927      936    +9     
=======================================
+ Hits          923      932    +9     
  Misses          4        4           

Flag	Coverage Δ
macos-latest	97.64% <100.00%> (+0.02%)	⬆️
ubuntu-latest	97.64% <100.00%> (+0.02%)	⬆️
windows-latest	95.51% <100.00%> (+0.04%)	⬆️

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[hugovk]

Fixes #366.

If you open this issue, you'll see there's already an open PR for this: #367.

Check next time, I'm getting so many duplicates that it wastes review time that can be spent elsewhere.

[hugovk]

"976.6 bytesK" is nonsense! 🙃