Fix fare calculation for combined interlined legs

[leonardehrenfried]

Summary

This fixes a problem when interlined legs area combined for fare calculation purposes: the fare products are associated to the wrong legs because the combined leg is not "deconstructed" into its parts when returning the results.

@binh-dam-ibigroup: this bugfix is relevant when Houston and SEPTA are going migrate the new Fares API.

Unit tests

Added.

Documentation

Javadoc added.

[codecov]

Codecov Report

All modified lines are covered by tests ✅

Comparison is base (bdfda72) 66.59% compared to head (941b2bd) 66.59%.
Report is 16 commits behind head on dev-2.x.

Additional details and impacted files

@@              Coverage Diff              @@
##             dev-2.x    #5408      +/-   ##
=============================================
- Coverage      66.59%   66.59%   -0.01%     
- Complexity     15285    15286       +1     
=============================================
  Files           1790     1790              
  Lines          69384    69392       +8     
  Branches        7307     7309       +2     
=============================================
+ Hits           46207    46210       +3     
- Misses         20700    20703       +3     
- Partials        2477     2479       +2     

Files	Coverage Δ
...r/ext/fares/impl/CombinedInterlinedTransitLeg.java	57.69% <100.00%> (+1.69%)	⬆️
...tripplanner/ext/fares/impl/DefaultFareService.java	92.19% <100.00%> (+0.11%)	⬆️
.../main/java/org/opentripplanner/model/plan/Leg.java	77.92% <ø> (ø)

... and 2 files with indirect coverage changes

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