[MOD-10926] Expose SVS Marked Deleted Vector Count via Statistics API

[meiravgri]

Overview

This PR exposes the number of marked deleted vectors in SVS through the VecSimIndexStatsInfo.numberOfMarkedDeleted field.

Key Changes:

Added virtual getNumMarkedDeleted() method to tiered index interface

• HNSW Tiered Implementation: Delegates to hnsw->VecSimIndexTombstone::getNumMarkedDeleted() (HNSW inherits from VecSimIndexTombstone)
• SVS Tiered Implementation: Returns SVSIndexBase::getNumMarkedDeleted()
• Updated VecSimTieredIndex::statisticInfo(): Now calls this->getNumMarkedDeleted() instead of using complex dynamic casting

Additional changes

Better marked deleted interface for SVS:

• Renamed changes_num → num_marked_deleted in SVSIndex to emphasize it only counts deletions
• Moved num_marked_deleted from SVSIndex to SVSIndexBase for easier access
• Added getNumMarkedDeleted() method to SVSIndexBase API
• Added virtual getNumMarkedDeleted() method to tiered index interface

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.64%. Comparing base (9fb223a) to head (7a976b9).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #796      +/-   ##
==========================================
- Coverage   96.66%   96.64%   -0.03%     
==========================================
  Files         126      126              
  Lines        7707     7712       +5     
==========================================
+ Hits         7450     7453       +3     
- Misses        257      259       +2     

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[github-actions]

Backport failed for 8.2, because it was unable to cherry-pick the commit(s).

Please cherry-pick the changes locally and resolve any conflicts.

git fetch origin 8.2
git worktree add -d .worktree/backport-796-to-8.2 origin/8.2
cd .worktree/backport-796-to-8.2
git switch --create backport-796-to-8.2
git cherry-pick -x 1c0ec77b9ce802a5023f376fcdebf824dc79c64b