Seed zero-value metrics for empty buckets in count-items

[delthas]

count-items only produced metric documents for buckets that hold objects. An empty bucket yielded no document in __infostore, so Scuba's deep health check (getAnyMetric) found nothing and returned 500, causing CloudServer to disable the quota service cluster-wide (ARTESCA-17063).

This seeds zero-value entries for the empty bucket, its account and its location in getObjectMDStats() when a bucket scan yields no objects, so every known resource always has a document in __infostore.

The bucket key is derived from the __usersbucket creation date, exactly like populated buckets (S3UTILS-131), so the seeded document matches the key quota lookups use. If that date is missing, the bucket is skipped with a warning rather than written under a key that lookups could never match.

Note: this addresses only the empty-bucket case. The new-bucket window (before the first count-items run) and the Scuba deep-health-check / alert behavior are tracked separately.

Issue: S3UTILS-224

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 45.62%. Comparing base (128b8c7) to head (9d957f3).
⚠️ Report is 4 commits behind head on development/1.

Additional details and impacted files

@@                Coverage Diff                @@
##           development/1     #378      +/-   ##
=================================================
+ Coverage          45.47%   45.62%   +0.15%     
=================================================
  Files                 88       88              
  Lines               6529     6544      +15     
  Branches            1372     1376       +4     
=================================================
+ Hits                2969     2986      +17     
+ Misses              3510     3508       -2     
  Partials              50       50              

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[claude]

LGTM

Review by Claude Code

[benzekrimaha]

object.keys(collRes.bucket).length === 0 does not necessarily mean the bucket is empty. It also happens when every entry failed processing, for example because of invalid content length or missing location configuration. We would then publish zero capacity for a non-empty bucket and potentially allow quota violations. can we track processing errors and only seed when the scan completed successfully without producing entries? we should also add a test for this case...