Handle BucketAlreadyExists races and bump arsenal for MPU afterEach fix

[benzekrimaha]

Summary

Fix MPU shadow-bucket creation races in cloudserver callers and bump arsenal to 8.4.20 (ARSN-613).

ARSN-613 makes MongoClientInterface.createBucket() return BucketAlreadyExists on concurrent duplicates instead of silent idempotence. Cloudserver must handle that contract:

• freshStartCreateBucket: continue addToUsersBucket / transient cleanup when the metastore entry already exists.
• getMPUBucket: on BucketAlreadyExists, fetch and return the existing MPU shadow bucket.

This fixes the intermittent afterEach failure in mpuVersion.js ("We encountered an internal error") when ListMultipartUploads races on MPU shadow bucket creation during test cleanup.

Paired with Arsenal PR #2672 (ARSN-613).

Issue: CLDSRV-919

[bert-e]

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[bert-e]

Incorrect fix version

The Fix Version/s in issue CLDSRV-919 contains:

• None

Considering where you are trying to merge, I ignored possible hotfix versions and I expected to find:

• 9.4.0

Please check the Fix Version/s of CLDSRV-919, or the target
branch of this pull request.

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[bert-e]

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[bert-e]

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.94%. Comparing base (055ae1b) to head (bda521d).
✅ All tests successful. No failed tests found.

Additional details and impacted files

Files with missing lines	Coverage Δ
lib/api/apiUtils/bucket/bucketCreation.js	93.49% <100.00%> (+0.10%)	⬆️
lib/services.js	87.63% <100.00%> (+0.19%)	⬆️

... and 2 files with indirect coverage changes

@@               Coverage Diff                @@
##           development/9.3    #6222   +/-   ##
================================================
  Coverage            84.93%   84.94%           
================================================
  Files                  206      206           
  Lines                13366    13374    +8     
================================================
+ Hits                 11353    11360    +7     
- Misses                2013     2014    +1     

Flag	Coverage Δ
file-ft-tests-null-compat	62.96% <40.00%> (-5.95%)	⬇️
kmip-ft-tests	28.28% <40.00%> (-0.01%)	⬇️
mongo-v0-ft-tests	69.59% <70.00%> (-0.04%)	⬇️
mongo-v1-ft-tests	69.60% <70.00%> (-0.02%)	⬇️
multiple-backend	36.75% <30.00%> (-0.02%)	⬇️
s3c-ft-tests-v0	64.09% <40.00%> (-0.03%)	⬇️
s3c-ft-tests-v0-null-compat	64.15% <40.00%> (-0.03%)	⬇️
s3c-ft-tests-v1	64.07% <40.00%> (-0.03%)	⬇️
sur-tests	35.81% <40.00%> (-0.83%)	⬇️
sur-tests-inflights	37.65% <40.00%> (-0.02%)	⬇️
unit	70.52% <100.00%> (+0.01%)	⬆️
utapi-v2-tests	34.54% <40.00%> (-0.01%)	⬇️

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