schemas: fix towed rolling stock structure

[woshilapin]

Seems like it was missed during the work on #9202, the schema was actually some sort of copy of RollingStock, but modifications were not done... 🤦

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All modified and coverable lines are covered by tests ✅

Project coverage is 39.07%. Comparing base (f7d17ed) to head (c7a7d26).
Report is 1 commits behind head on dev.

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@@             Coverage Diff              @@
##                dev    #9436      +/-   ##
============================================
- Coverage     39.07%   39.07%   -0.01%     
  Complexity     2270     2270              
============================================
  Files          1308     1308              
  Lines         99249    99249              
  Branches       3315     3315              
============================================
- Hits          38786    38777       -9     
- Misses        58499    58508       +9     
  Partials       1964     1964              

Flag	Coverage Δ
core	74.99% <ø> (ø)
editoast	71.98% <ø> (-0.04%)	⬇️
front	10.32% <ø> (ø)
gateway	2.50% <ø> (ø)
osrdyne	3.29% <ø> (ø)
railjson_generator	87.49% <ø> (ø)
tests	86.71% <ø> (ø)

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[leovalais]

Yup, indeed, good catch 👀

[EthanPERRUZZA]

I would have add some kind of comment somewhere (maybe on this PR?) to know why we moved to NonNegativeFloat here and not above.
Or it might be worth to also substitute the occurences of PositiveFloat above as it shouldn't have any impact and will make the code more harmonious.
Otherwise, LGTM