MJCF to SDFormat: Fix index generation for visuals and collisions

[azeey]

🦟 Bug fix

Summary

The index generation was not working properly because the variables NUMBER_OF_VISUAL and NUMBER_OF_COLLISION were not being incremented. The function prefix_name_with_index does increment the passed index, but it's incrementing a copy since integers are immutable in python.

Checklist

• [x] Signed all commits for DCO
• [x] Added tests
• [ ] Updated documentation (as needed)
• [ ] Updated migration guide (as needed)
• [ ] Consider updating Python bindings (if the library has them)
• [x] codecheck passed (See contributing)
• [x] All tests passed (See test coverage)
• [ ] While waiting for a review on your PR, please help review another open pull request to support the maintainers

Note to maintainers: Remember to use Squash-Merge and edit the commit message to match the pull request summary while retaining Signed-off-by messages.

[codecov[bot]]

Codecov Report

Merging #97 (d5098cc) into main (0d7ed5f) will increase coverage by 0.01%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main      #97      +/-   ##
==========================================
+ Coverage   95.12%   95.14%   +0.01%     
==========================================
  Files          24       24              
  Lines        1170     1173       +3     
==========================================
+ Hits         1113     1116       +3     
  Misses         57       57              

Impacted Files	Coverage Δ
sdformat_mjcf/src/sdformat_mjcf/utils/sdf_utils.py	90.98% <ø> (-0.08%)	:arrow_down:
.../sdformat_mjcf/mjcf_to_sdformat/converters/link.py	100.00% <100.00%> (ø)

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