Fix default class and names so generated xml doesn't contain `/`

[azeey]

🦟 Bug fix

Summary

I tried to convert dm_control/dm_control/suite/dog.xml and encountered an error

FileNotFoundError: Line 166: error while parsing element <mesh>: during initialization of attribute 'file' of element <mesh>: [Errno 2] No such file or directory: 'dog_assets\\/BONEC_1.stl'

The problem seem is that we are telling pymjcf to escape / characters and that is interfering with the slash in file paths. The solution here is to go back to ensure that the / is not generated in our output MJCF files.

Checklist

• [ ] Signed all commits for DCO
• [ ] Added tests
• [ ] Updated documentation (as needed)
• [ ] Updated migration guide (as needed)
• [ ] Consider updating Python bindings (if the library has them)
• [ ] codecheck passed (See contributing)
• [ ] 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

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[codecov[bot]]

Codecov Report

:exclamation: No coverage uploaded for pull request base (main@60beb59). Click here to learn what that means. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main      #94   +/-   ##
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  Coverage        ?   95.11%           
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  Files           ?       24           
  Lines           ?     1167           
  Branches        ?        0           
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  Hits            ?     1110           
  Misses          ?       57           
  Partials        ?        0           

Impacted Files	Coverage Δ
...sdformat_mjcf/mjcf_to_sdformat/mjcf_to_sdformat.py	100.00% <100.00%> (ø)
...sdformat_mjcf/sdformat_to_mjcf/converters/joint.py	97.56% <100.00%> (ø)
...sdformat_mjcf/sdformat_to_mjcf/sdformat_to_mjcf.py	100.00% <100.00%> (ø)

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