Fix empty snapshot loading and region cleanup

[shuwenwei]

Summary

This PR fixes two independent region migration cleanup issues:

• Allow snapshot loading to continue for empty regions whose snapshots do not contain seq or unseq data directories. The loader still reports a missing snapshot root as an error, but treats an existing snapshot with no data files as an empty region snapshot.
• Skip TableDiskUsageIndex removal for tree-model regions, and complete remove operations immediately when no table disk usage writer exists.

Root Cause

There are two separate root causes:

• Empty regions may not create seq/unseq data directories before migration, but the snapshot loader previously required at least one of those directories to exist.
• Tree-model regions are never registered in TableDiskUsageIndex, but delete-region cleanup still attempted to remove their table disk usage index entry. Since no writer exists for those regions, the remove future could remain incomplete until timeout.

[codecov]

Codecov Report

❌ Patch coverage is 24.13793% with 22 lines in your changes missing coverage. Please review.
✅ Project coverage is 40.67%. Comparing base (3d98ea5) to head (4675517).
⚠️ Report is 6 commits behind head on master.

Files with missing lines	Patch %	Lines
...rageengine/dataregion/snapshot/SnapshotLoader.java	26.66%	11 Missing ⚠️
...utils/tableDiskUsageIndex/TableDiskUsageIndex.java	0.00%	11 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #17880      +/-   ##
============================================
- Coverage     40.85%   40.67%   -0.18%     
- Complexity     2611     2620       +9     
============================================
  Files          5201     5244      +43     
  Lines        354010   362374    +8364     
  Branches      45313    46653    +1340     
============================================
+ Hits         144626   147402    +2776     
- Misses       209384   214972    +5588     

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[sonarqubecloud]

Quality Gate passed

Issues
3 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
2.1% Duplication on New Code

See analysis details on SonarQube Cloud