Fix multipart S3 composite checksum suffix

[SergioChan]

Summary

• append the multipart part-count suffix (-N) to computed multipart S3 checksums when ChecksumAlgorithm is used
• add a multipart regression test that verifies Moto now returns the AWS-style composite checksum format for ChecksumSHA256
• keep behavior scoped to multipart completion checksum generation only

Testing

• python -m pytest tests/test_s3/test_s3_multipart.py -k "checksum_has_composite_suffix"

Related

• Fixes Multipart S3 uploads - composite checksums are not given the -X suffix #8762

[SergioChan]

Pushed a follow-up commit to address the CI failure in test_copy_key_with_args__using_multipart.

What changed

• Updated multipart checksum formatting so the -<part_count> suffix is only added when there are multiple parts.
• Single-part uploads now return the base checksum value without -1.

Validation

• python -m pytest tests/test_s3/test_s3_copyobject.py -k test_copy_key_with_args__using_multipart -q
• python -m pytest tests/test_s3/test_s3_multipart.py -k test_multipart_upload_checksum_has_composite_suffix -q

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.12%. Comparing base (ee7a095) to head (f7cc0bb).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #9849   +/-   ##
=======================================
  Coverage   93.12%   93.12%           
=======================================
  Files        1314     1314           
  Lines      119430   119432    +2     
=======================================
+ Hits       111224   111227    +3     
+ Misses       8206     8205    -1     

Flag	Coverage Δ
servertests	29.21% <0.00%> (+<0.01%)	⬆️
unittests	93.10% <100.00%> (+<0.01%)	⬆️

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[SergioChan]

Thanks @codecov[bot] for the review feedback. I’m working through the requested changes and will follow up with an updated patch/answers shortly.