core: pathfinding: fix pathfinding class to handle overlapping points

[eckter]

We didn't always handle this case properly, we kind of only did sometimes accidentally handle it. Caused issues in #10200.

(this pathfinding class is a mess)

[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.80%. Comparing base (a90a394) to head (3738f3d).
Report is 1 commits behind head on dev.

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Additional details and impacted files

@@            Coverage Diff             @@
##              dev   #10512      +/-   ##
==========================================
- Coverage   81.81%   81.80%   -0.01%     
==========================================
  Files        1073     1073              
  Lines      106718   106718              
  Branches      730      730              
==========================================
- Hits        87306    87301       -5     
- Misses      19373    19378       +5     
  Partials       39       39              

Flag	Coverage Δ
editoast	74.27% <ø> (-0.02%)	⬇️
front	89.34% <ø> (ø)
gateway	2.18% <ø> (ø)
osrdyne	3.28% <ø> (ø)
railjson_generator	87.50% <ø> (ø)
tests	87.05% <ø> (ø)

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