Fix incorrect assertion in BM_VecSimBasics::UpdateAtBlockSize benchmark

[meiravgri]

The UpdateAtBlockSize benchmark contained an incorrect assertion that expected metadata containers to shrink by exactly one block size during delete operations:

assert(index->indexCapacity() == index_cap - BM_VecSimGeneral::block_size);

Why it was broken

1. For Flat indexes: After PR [MOD-10559] Decouple the shrinking and growing logic of large containers in Flat and HNSW #753's conservative resize strategy, metadata containers (idToLabelMapping) only shrink when indexCapacity() >= (indexSize() + 2 * blockSize), not at exact blocksize boundaries
2. For HNSW indexes: indexCapacity() returns the capacity of vector containers, not metadata containers. While the assertion didn't fail, it was testing vector container capacity instead of the intended metadata container capacity, making it ineffective at validating the benchmark's actual purpose

What was fixed

• Introduced indexMetaDataCapacity() method: Returns the actual capacity of metadata containers (idToLabelMapping for Flat, idToMetaData for HNSW, combined capacities for tiered indexes)
• Updated benchmark assertions: Changed from expecting capacity reduction to verifying capacity stability during delete/add cycles, which correctly tests the conservative resize behavior introduced in PR [MOD-10559] Decouple the shrinking and growing logic of large containers in Flat and HNSW #753

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.66%. Comparing base (9f1fc61) to head (f2b936c).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #819   +/-   ##
=======================================
  Coverage   96.66%   96.66%           
=======================================
  Files         126      126           
  Lines        7368     7379   +11     
=======================================
+ Hits         7122     7133   +11     
  Misses        246      246           

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[GuyAv46]

No need to lock here?

[meiravgri]

i took reference from indexCapacity()

[meiravgri]

from what i see indexCapacity() is also only used for testing

[GuyAv46]

Can this be moved to tiered_index.h?

[meiravgri]

same

[github-actions]

Backport failed for 0.6, because it was unable to cherry-pick the commit(s).

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

git fetch origin 0.6
git worktree add -d .worktree/backport-819-to-0.6 origin/0.6
cd .worktree/backport-819-to-0.6
git switch --create backport-819-to-0.6
git cherry-pick -x f24b65afa98f99352f7c1f384e24f2c2a4cbaad1

[github-actions]

Backport failed for 0.7, because it was unable to cherry-pick the commit(s).

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

git fetch origin 0.7
git worktree add -d .worktree/backport-819-to-0.7 origin/0.7
cd .worktree/backport-819-to-0.7
git switch --create backport-819-to-0.7
git cherry-pick -x f24b65afa98f99352f7c1f384e24f2c2a4cbaad1

[github-actions]

Backport failed for 0.8, because it was unable to cherry-pick the commit(s).

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

git fetch origin 0.8
git worktree add -d .worktree/backport-819-to-0.8 origin/0.8
cd .worktree/backport-819-to-0.8
git switch --create backport-819-to-0.8
git cherry-pick -x f24b65afa98f99352f7c1f384e24f2c2a4cbaad1

[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-819-to-8.2 origin/8.2
cd .worktree/backport-819-to-8.2
git switch --create backport-819-to-8.2
git cherry-pick -x f24b65afa98f99352f7c1f384e24f2c2a4cbaad1