fix: Fix AMVF finding with negative cov

[andiwand]

AMVF vertex finding dropped vertex candidates because isMergedVertex would return true for negative covariance matrices.

The reason why these matrices are negative definite is still unclear but we should not consider them as merged vertices by default I think.

This was the reason why we experienced noisy AMVF physmon diff for mostly unrelated changes because these conditions would flip flop on small numerical changes. A small reproducible example can be found in #2372

[codecov]

Codecov Report

Merging #2374 (5540b90) into main (543bd5a) will decrease coverage by 0.01%.
The diff coverage is 14.28%.

@@            Coverage Diff             @@
##             main    #2374      +/-   ##
==========================================
- Coverage   49.57%   49.57%   -0.01%     
==========================================
  Files         453      453              
  Lines       25513    25514       +1     
  Branches    11707    11707              
==========================================
  Hits        12649    12649              
- Misses       4579     4580       +1     
  Partials     8285     8285              

Files Changed	Coverage Δ
...clude/Acts/Vertexing/AdaptiveMultiVertexFinder.ipp	40.81% <14.28%> (-0.14%)	⬇️

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[github-actions]

📊: Physics performance monitoring for 5540b90

Full contents

physmon summary

• ✅ CKF truth_smeared
• ✅ IVF truth_smeared
• ✅ AMVF truth_smeared
• ✅ Track Summary CKF truth_smeared
• ✅ Seeding truth_estimated
• ✅ CKF truth_estimated
• ✅ IVF truth_estimated
• ✅ AMVF truth_estimated
• ✅ Track Summary CKF truth_estimated
• ✅ Seeding seeded
• ✅ CKF seeded
• ✅ IVF seeded
• ✅ AMVF seeded
• ✅ Track Summary CKF seeded
• ✅ Seeding orthogonal
• ✅ CKF orthogonal
• ✅ IVF orthogonal
• ✅ AMVF orthogonal
• ✅ Track Summary CKF orthogonal
• ✅ Ambisolver seeded
• ✅ Ambisolver orthogonal
• ✅ Seeding ttbar
• ✅ CKF ttbar
• ✅ Ambisolver
• ✅ AMVF ttbar
• ✅ Truth tracking (GSF)
• ✅ Truth tracking
• ✅ Particles inital fatras
• ✅ Particles final fatras
• ✅ Particles inital geant4
• ✅ Particles final geant4

Invalidated by push of a541819

[acts-project-service]

🔴 Athena integration test results

🔴 Some tests have failed!

Please investigate the pipeline!

status	job
🟢	run_unit_tests
🟢	run_ci_tests: ../athena/AtlasTest/CITest/test/ActsWorkflow.sh
🟢	run_ci_tests: ../athena/AtlasTest/CITest/test/ActsValidateTracks.sh
🟢	run_ci_tests: ../athena/AtlasTest/CITest/test/ActsValidateActsCoreSpacePoints.sh
🟢	run_ci_tests: ../athena/AtlasTest/CITest/test/ActsValidateActsSpacePoints.sh
🔴	run_ci_tests: ../athena/AtlasTest/CITest/test/ActsValidateSeeds.sh
🟢	run_ci_tests: ../athena/AtlasTest/CITest/test/ActsValidateOrthogonalSeeds.sh
🟢	run_ci_tests: ../athena/AtlasTest/CITest/test/ActsValidateClusters.sh
🟢	run_ci_tests: ../athena/AtlasTest/CITest/test/ActsPersistifyEDM.sh
🟢	run_ci_tests: ../athena/AtlasTest/CITest/test/ActsKfRefitting.sh
🟢	run_ci_tests: python3 ../athena/Tracking/Acts/ActsGeometry/test/ActsExtrapolationAlgTest.py
🟢	run_ci_tests: python3 ../athena/Tracking/Acts/ActsGeometry/test/ActsITkTest.py
🟢	run_workflow_tests_run4_mc
🔴	run_workflow_tests_run2_mc
🔴	run_workflow_tests_run2_data
🟢	run_workflow_tests_run3_mc
🔴	run_workflow_tests_run3_data