Verify file size

> When the input CSV doesn't have a checksum column we might fail to download the entire file without notifying the user. Could you check the [response headers](https://requests.readthedocs.io/en/latest/user/quickstart/#response-headers) for
[Content-Length](https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Content-Length) and compare that value to the size of the full size image file? Often web servers will set this response header so it would be nice to check it when this value is sent in the response.

_Originally posted by @johnbradley in https://github.com/Imageomics/cautious-robot/pull/8#discussion_r1631663496_

See also discussion following.

Trace the Python download and verification path for CSV-sourced full-size images, starting with where response headers and checksum handling are processed. Compare Content-Length with the downloaded file size when the header is present, and verify that incomplete downloads notify the user without breaking cases where the header is absent.

Written by the indexing model from the issue text.