Raise object-store test coverage above 90% with new edge-case tests

[predatorray]

Adds unit and dispatch-level tests around the core object store, lifting
aggregate line coverage from 90.0% to 91.3% (instruction 90.3%->91.8%,
branch 80.3%->83.3%). No production behavior changed — every edge case
exercised behaved correctly, so no fixes were required.

New / extended coverage:

• BoxEngine (94.1%->99.3%): null user-metadata puts, multipart argument
  validation (create/upload/copy/complete/abort), part supersession,
  object-ACL read/replace + not-found, range-GET boundary resolution
  (suffix clamping, neither-bound, empty object), prefix/reverse scan
  corners, recovery that re-opens flushed SSTables, empty dropSyrups.
• SyrupReader (89.8%->100%): chunk-too-short and per-part length/CRC
  mismatch detection, invalid range bounds, under-producing range reads,
  and downstream-write IOException wrapping.
• SSTableReader (88.6%->94.7%): empty-ledger rejection, scans past the
  key range, exhausted-iterator NoSuchElement.
• ManifestEdit / MultipartUploadState (->100%): null-collection
  normalization, fencing-token and PartUpsert validation, builder setters.
• CandyLocator / CandyKey (->100%): tombstone accessors, null-field and
  DELETE-with-parts validation, ofUtf8 empty rejection, hashCode/toString.
• NodeRequestHandler (90.5%->95.4%): ListMultipartUploads prefix/marker/
  paging, ListParts marker/paging/not-found, object-ACL get/set round trip
  and malformed-grant errors, same-partition UploadPartCopy.

https://claude.ai/code/session_016STTyQPGPpXbUkwPMfV7gS

[github-actions]

✅ S3 compatibility gate passed

• mode: gate
• result: 164 passed in 72.43s (0:01:12)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.66%. Comparing base (2e99cc5) to head (a636e58).

Additional details and impacted files

@@             Coverage Diff              @@
##               main      #22      +/-   ##
============================================
+ Coverage     82.20%   83.66%   +1.45%     
  Complexity      677      677              
============================================
  Files           161      161              
  Lines          8874     8874              
  Branches       1330     1330              
============================================
+ Hits           7295     7424     +129     
+ Misses         1048      970      -78     
+ Partials        531      480      -51     

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.