CLDSRV-898: handle checksums in CompleteMultipartUpload

[leif-scality]

• Calculate and compare the final object checksum with the one sent by the headers
• Check that all parts have the correct checksum and checksum type
• stores the final checksum when FULL_OBJECT (COMPOSITE are going to be stored by https://scality.atlassian.net/browse/S3C-10399)

[claude]

LGTM

Review by Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 91.32948% with 15 lines in your changes missing coverage. Please review.
✅ Project coverage is 85.41%. Comparing base (8754b9a) to head (27b4a43).
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
lib/api/completeMultipartUpload.js	89.51%	13 Missing ⚠️
lib/api/apiUtils/integrity/validateChecksums.js	95.91%	2 Missing ⚠️

Additional details and impacted files

Files with missing lines	Coverage Δ
lib/api/apiUtils/integrity/validateChecksums.js	98.79% <95.91%> (-0.26%)	⬇️
lib/api/completeMultipartUpload.js	89.90% <89.51%> (+0.55%)	⬆️

... and 3 files with indirect coverage changes

@@                                      Coverage Diff                                       @@
##           improvement/CLDSRV-902-calculate-final-checksum-from-parts    #6170      +/-   ##
==============================================================================================
+ Coverage                                                       85.26%   85.41%   +0.14%     
==============================================================================================
  Files                                                             208      208              
  Lines                                                           13889    14039     +150     
==============================================================================================
+ Hits                                                            11843    11991     +148     
- Misses                                                           2046     2048       +2     

Flag	Coverage Δ
file-ft-tests	69.52% <64.16%> (+0.60%)	⬆️
kmip-ft-tests	27.92% <3.46%> (-0.26%)	⬇️
mongo-v0-ft-tests	70.72% <64.16%> (+0.64%)	⬆️
mongo-v1-ft-tests	70.63% <64.16%> (+0.56%)	⬆️
multiple-backend	36.17% <3.46%> (-0.35%)	⬇️
sur-tests	35.18% <3.46%> (-0.34%)	⬇️
sur-tests-inflights	38.12% <47.97%> (+0.71%)	⬆️
unit	72.22% <90.17%> (+0.20%)	⬆️
utapi-v2-tests	35.54% <47.97%> (+0.89%)	⬆️

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