Call shrink to fit for id to label mapping in flat index

[alonre24]

Describe the changes in the pull request

id_to_label_mapping in brute force index is a std::vector, and whenever we resize it, its capacity is determined according to this container's policy. Then, when we add a block and call for resize, the capacity is doubled (rather than increased by block size). The fix is calling shring_to_fit() which deallocates unnecessary memory from the container (so its capacity matches its size).

Mark if applicable

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

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: -0.06 ⚠️

Comparison is base (9f38780) 96.99% compared to head (b6e9095) 96.93%.

❗ Current head b6e9095 differs from pull request most recent head 5e0b738. Consider uploading reports for the commit 5e0b738 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #362      +/-   ##
==========================================
- Coverage   96.99%   96.93%   -0.06%     
==========================================
  Files          63       63              
  Lines        3559     3561       +2     
==========================================
  Hits         3452     3452              
- Misses        107      109       +2     

Impacted Files	Coverage Δ
src/VecSim/algorithms/brute_force/brute_force.h	100.00% <100.00%> (ø)

... and 1 file with indirect coverage changes

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