Synchronize properly repair/insert test

[alonre24]

Describe the changes in the pull request

Completion for #546 where we added the index guard mechanism to make parallel insertion into HNSW thread safe with respect to data blocks resize that requires exclusive lock.

Here we add the missing protection to the repair operation with mutual lock while insertion is running.
Also, setting a lower block size in the test to better expose the failure, and apply the common synchronization logic to another parallel test.

Mark if applicable

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

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.93%. Comparing base (1381f64) to head (4ace1e7).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #555   +/-   ##
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  Coverage   96.93%   96.93%           
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  Files         100      100           
  Lines        5287     5287           
=======================================
  Hits         5125     5125           
  Misses        162      162           

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