BF: Seek file data to 0 before every retry in request(), not just for retryable status codes

[yarikoptic]

• Closes Validation error related to content-md5 when uploading OME-Zarr #1821

@dstansby please try

claude's slopsplaining:

Previously, data.seek(0) was only called inside the retry_statuses/retry_if branch, which handles specific HTTP response codes (429, 500-504, and zarr-specific conditions). When a ConnectionError occurred mid-upload (e.g., connection dropped during a multi-minute large zarr chunk transfer), tenacity caught the exception and retried, but the file pointer was left at whatever position the read was interrupted.

On retry, requests computed Content-Length as (file_size - current_position) and sent only the tail of the file. S3 received partial data whose MD5 didn't match the Content-MD5 header (computed from the full file), resulting in a BadDigest 400 error. This error was also not retried (not in RETRY_STATUSES, not matched by retry_if), so the upload failed permanently.

The fix uses tenacity's before_sleep callback to seek the file data back to position 0 before every retry, regardless of whether the retry was triggered by ConnectionError, HTTPError, or a retryable status code.

Evidence from two independent logs (issue #1821):

• Linux: 188 level-0 zarr chunks failed (large, ~6 min upload each), 67 level-2 chunks succeeded (small, fast upload) -- all with "succeeded after 1 retry" + BadDigest
• Windows local disk: 187 level-0 failures after 2-8 retries, 68 level-1 successes -- ruling out NFS/filesystem as the cause
• Both logs show "Resetting dropped connection: dandiarchive.s3.amazonaws.com" confirming ConnectionErrors preceded the BadDigest errors

[codecov]

Codecov Report

❌ Patch coverage is 66.66667% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 75.12%. Comparing base (5f03d9b) to head (1a35018).

Files with missing lines	Patch %	Lines
dandi/dandiapi.py	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1824      +/-   ##
==========================================
- Coverage   75.12%   75.12%   -0.01%     
==========================================
  Files          84       84              
  Lines       11930    11931       +1     
==========================================
  Hits         8963     8963              
- Misses       2967     2968       +1     

Flag	Coverage Δ
unittests	75.12% <66.66%> (-0.01%)	⬇️

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