[SVS] Benchmark vectors add during training

[rfsaliev]

Added benchmark for updating Tiered SVS Index in parallel with training job

This PR contains simple benchmark to measure Tiered SVS Index updates executed in parallel with background SVS training.

Which issues this PR fixes

1. #...
2. MOD...

Main objects this PR modified

1. BM VecSim SVS
2. ...

Mark if applicable

• This PR introduces API changes
• This PR introduces serialization changes

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Note

Low Risk
Benchmark-only change that adds a new test scenario and does not affect production SVS/index behavior; main risk is increased/variable benchmark runtime or flakiness due to thread scheduling.

Overview
Adds a new SVS tiered-index benchmark that measures AddVector performance while the backend is actively training.

Introduces BM_AddVectorsDuringTraining, which fills to a configurable training threshold, starts the async thread pool, then continuously adds more vectors during the training job (using real-time measurement and configurable thread counts), while avoiding tiered updates via an intentionally high update threshold.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.12%. Comparing base (8819850) to head (ccb4b28).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #910      +/-   ##
==========================================
+ Coverage   97.09%   97.12%   +0.02%     
==========================================
  Files         129      129              
  Lines        7500     7500              
==========================================
+ Hits         7282     7284       +2     
+ Misses        218      216       -2     

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