Prevent interrupted FNL downloads leaving truncated files

[aethr]

Description

Fixes #81.

The upstream FNL file for 2022-11-01 00Z is intact: a fresh download subsets cleanly with wgrib2. The failure was caused by a truncated local copy:

• download_file wrote directly to the final filename. Cleanup only ran on a RequestException, so a download that was killed part way (container stopped, Ctrl-C, joblib terminating workers after another download failed) left a partial file in place.
• setup_for_wrf.py only checks whether an FNL file exists before skipping its download, so subsequent runs reused the truncated file and wgrib2 failed with read outside of file, bad grib file.

Six truncated files were found in a local FNL cache, each roughly 35–50% of the expected size.

This PR downloads to a <name>.part file, verifies its size against Content-Length (when the response isn't content-encoded), and then moves it into place with os.replace. The .part file is always removed on failure or interruption, so the final path only ever holds a complete file.

Checklist

Please confirm that this pull request has done the following:

• Tests added
• Documentation added (where applicable)
• Changelog item added to changelog/

Notes

Existing truncated files in FNL caches are not detected by this change and need to be deleted manually.

🤖 Generated with Claude Code

[prayner]

test_fetch_fnl.py:29 why testing for an empty list rather than a null
path?

[aethr]

test_fetch_fnl.py:29 why testing for an empty list rather than a null path?

Stylistically, there is consistency with the initial test, and all the following tests:

# expected file exists
assert list(tmp_path.iterdir()) == [result]

# no file exists
assert list(tmp_path.iterdir()) == []

In the case of the tests where no file is expected, this check covers a lot of scenarios:

• the file wasn't downloaded to the expected filename
• the file wasn't downloaded to any filename
• a partial file isn't present

I don't mind also adding the explicit "expected file doesn't exist" assertion if you think it makes it clearer, or adding a comment to these tests.

[aethr]

Added better comments to the assertions here.