[0.6] [MOD-11237] Block Size Boundary Oscillation Benchmark (#768

[meiravgri]

backport of #768 to 0.6

Add virtual indexCapacity() API to VecSimIndexInterface

API Changes

• BruteForce: Capacity determined based on idToLabelMapping size (aligned with its capacity)
• HNSW: Rename existing getIndexCapacity and change it to override the new virtual API

[codecov]

Codecov Report

❌ Patch coverage is 33.33333% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 94.99%. Comparing base (520531a) to head (a10bc71).
⚠️ Report is 1 commits behind head on 0.6.

Files with missing lines	Patch %	Lines
src/VecSim/algorithms/brute_force/brute_force.h	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              0.6     #773      +/-   ##
==========================================
- Coverage   95.04%   94.99%   -0.06%     
==========================================
  Files          60       60              
  Lines        3432     3434       +2     
==========================================
  Hits         3262     3262              
- Misses        170      172       +2     

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