[backend] fix non-idempotent operation during concurrent stream sync (#15207)

[OctaveLaventure]

Proposed changes

• prevent relation duplication with parallel workers on ADD operations

Related issues

• Ensure reports rel_object.internal_id are deduplicated #15207

Checklist

• I consider the submitted work as finished
• I tested the code for its functionality
• I wrote test cases for the relevant uses case (coverage and e2e)
• I added/update the relevant documentation (either on github or on notion)
• Where necessary I refactored code to improve the overall quality

Further comments

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 26.11%. Comparing base (8dd5e6c) to head (312deef).
⚠️ Report is 23 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #15582      +/-   ##
==========================================
- Coverage   33.52%   26.11%   -7.42%     
==========================================
  Files        3167     3190      +23     
  Lines      214695   217696    +3001     
  Branches    39431    37264    -2167     
==========================================
- Hits        71978    56847   -15131     
- Misses     142717   160849   +18132     

Flag	Coverage Δ
opencti-client-python	45.60% <ø> (ø)
opencti-front	3.03% <ø> (+0.09%)	⬆️
opencti-graphql	52.73% <100.00%> (-17.09%)	⬇️

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