Pre-seed zero-value metrics for empty buckets

[delthas]

When all buckets are empty (0 objects), count-items produces no
bucket metric documents in __infostore. This is because
getObjectMDStats() only populates dataMetrics.bucket entries when
the MongoDB cursor yields objects — with 0 objects, the cursor
yields nothing, _processEntryData() is never called, and
dataMetrics.bucket remains {}. The flatMap in
updateStorageConsumptionMetrics() then produces zero documents,
and the atomic collection rename leaves __infostore with only
the countitems marker.

Downstream, Scuba's deep health check queries __infostore for any
metric document and crashes when it finds none, causing CloudServer
to mark the quota service as unavailable (ARTESCA-17063).

Pre-seed dataMetrics.bucket with zero-value entries for every known
bucket in CountManager.addWork(), using consolidateDataMetrics() to
create the canonical zero structure. When workers return results for
non-empty buckets, consolidation adds on top of these zeros. For
empty buckets, the zero entries survive through to
updateStorageConsumptionMetrics, ensuring every bucket gets a
document in __infostore.

Issue: S3UTILS-224

[bert-e]

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 43.65%. Comparing base (378f68f) to head (c954e09).

Additional details and impacted files

@@                 Coverage Diff                  @@
##           development/1.17     #378      +/-   ##
====================================================
+ Coverage             43.62%   43.65%   +0.02%     
====================================================
  Files                    84       84              
  Lines                  5973     5976       +3     
  Branches               1255     1256       +1     
====================================================
+ Hits                   2606     2609       +3     
  Misses                 3321     3321              
  Partials                 46       46              

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